Opening Remarks
This being a Beta feedback thread I'll be concentrating mostly on my opinion of the games weaknesses rather than its strengths. Where I use acronyms they'll be explained in their first use. For the purposes of this review, I was considering the game being released "as is", many of the negative remarks may well be on subjects that the full release fixes anyway (and I didn't go parsing through all patch notes and upcoming builds to filter those ones out).
Don't let my criticisms mislead you though, I've thoroughly enjoyed my time playing the game and would recommend it to any "explore, expand, exploit, and exterminate" (4x) turn-based strategy fan.

Review Criteria
This review is based off a number of games, each using a different sized galaxy available in the Beta client. I didn't however play the story mode option in any of the games, preferring Classic so I cannot honestly review that aspect. In one game, to test several features I edited the save-game files to give myself a scandalously large amount of credits, but all other games were playing without cheating.
So, before I go all critical, there are a few strengths I liked so much I just have to give them a special mention. And remember folks, I present nothing here as fact, merely opinion (unless I specifically state otherwise).
Strengths
1) Inspiration
Obviously Horizon has been heavily influenced by Masters Of Orion 2 (MOO2). I count that as a real strength given my belief that MOO2 even today is one of the more enjoyable 4x games on the market. *FACT* Released 17 years ago now, it's an amazing testament to MOO2 (and MOO1 as well) that it still often referenced in contemporary game reviews. So, the game, for me, scored big points there. Without even reading the manual I was able to "get the jist of it all" on my very first game.
In particular, the UI (despite one or two issues) made me feel right at home.
2) Micromanagement
Hardly any. Loved it. Even when I had built a sprawling empire across most of the largest galaxy the Beta client allows, I was very rarely bogged down in micromanagement. This is an especially important strength of the game as it's a common failing in other games (for me, Hearts of Iron 3 would be a prime example of this, albeit being a vastly more complex game).
3) Tactical Combat
Really enjoyable to watch (more on this in the weaknesses).
4) Ship Design
The influence of the MOO series was especially evident here and it was a solid, if a little sparse, feature.
5) Planet Management
As with Ship Design, so far as the Planet Management went, it was well done, quite intuitive and easily managed.
6) Strengths Summary
While there are more things I could list here, they'd be mostly variations on themes already discussed, suffice it say though that I found the game overall very enjoyable, and the Beta client itself never missed a beat.
Weaknesses
1) Difficulty
In each of the games (including the first which I jumped into without reading the manual), I won handily, save one in which I was crushingly (and fairly promptly) defeated. Overall however, I would say the difficulty of the game is fairly tame, and the lack of difficulty level selections is an obvious missing feature.
2) Races
Two main criticisms I'd make here. Firstly, the default AI settings assigned to each race seemed to leave little or no room for individual or random behaviour on the part of the computer players. In all games, every last one, I was quickly and without any casus belli on my part that I could see at war with the Varaians, Tantik and Har'kan. Similarly, I ended up in Alliance with the Barsig, Kuntari (I'd really suggest a name change there) and Lezgoon.
The lack of a "Set to Random" option here under the AI Settings in the Opponents screen was also a disappointment, as it made each individual computer player fairly predictable (after the 3rd or 4th game I knew I would be soon at war with the usual crowd).
3) Tactical Combat
A bit slow for my tastes, I ended up pretty much always selecting Auto on the Fast Forward button, particularly if the Har'kan were involved as their ships jump ability fairly halved the rate of combat progression. Still, the saving grace for me here was the Auto and Fast feature.
Perhaps some shorter animation times may help here, though for large battles I think manual play will always be too slow for me.
4) Ship Movement - Strategic
This was one of the features I least liked. Moving ships across the galaxy was tediously slow, advancements in engine technology seemed to make no difference at all to their interstellar pace of movement, and the tendency for a fleet to spread out over multiple sectors made the galactic map unnecessarily cluttered and more difficult to view than it needed to be. This is especially true of fleets of mixed ship size.
Some form of formation flying is needed here I believe, both for aesthetic reasons and for tactical reasons discussed below. Also, advance in propulsion technology need to make a difference in how quickly my fleets slowly plod their way across the galaxy map.
5) Ship Movement - Tactical
Many a brave Terran lost his (or her) life to an early arrival in unfriendly space. The natural consequence of a fleet spreading itself out over several sectors meant that ships could arrive in combat almost one by one, turns apart, and be consequently picked off one by one by a defending force.
When making long journeys (and did I mention that they were very noticeably long?) into dangerous space I would often have the fleet stop one sector before, regather itself (especially via the "Orbit" command if they stopped in a sector with planets), before proceeding into the combat zone.
Perhaps a second element I noticed was a bug, but I also occasionally found when sending multiple fleets into a single sector that one or the other would sit fat, dumb and happy for a number of turns before obeying an Orbit order. Fleets also seemed to have issues being given a Bombard order when not already orbiting the chosen planet.
As with strategic ship movement, some form of formation flying would be most welcome here.
6) Ship Design
It just felt a little too generic. There weren't any really unusual designs I could make (and there was naught to find via archaeological digs on planets). As I mentioned in the Strengths section, the system is good so far as it goes, I just wish it went a little further.
Also, I missed MOO's option of allowing for extra space at the expense of a higher ship building time and cost.
7) Ground Combat
Though I'm a "Kill Em All" kinda guy, and quite adverse to sharing my planets with pesky aliens, I did indulge in a little ground combat to test the systems out. This was even sparser than MOO's options here, as all I got to do was watch two lots of soldier numbers count down.
Ultimately, it'd be nice to see more 4x space games embrace some more in depth tactical options when it comes to ground combat (one I played many years back actually had you place buildings in specific sectors on a planet, and invasions of planets involved moving armies over that planetary map, with damage resulting to specific buildings as combat proceeded).
In the absence of more options for ground combat, a least a little more eye candy in the current system would be nice.

Not really enough of it I thought. The highest level structures, were still only 3 or 4 deep, which didn't allow for the sort of hyper-specialisation I was hoping for.
For example, every non-capital planet I had, had Trade Centre, Hydroponic Farms, Factories and Research Centre. I was able to skip Entertainment entirely without any ill-effects, meaning that I didn't really ever have specialised planets. It would be good to see another level of building within each category, to offer a choice to specialise very highly in one area at the sacrifice of others.
It would also be good to see Entertainment become more meaningful.
9) Finances
Seems a tad bit too easy here. Generating a net income of around 100bc was pretty easily achieved, and this is sufficient to keep a planetary system with 5 planets pumping out paid-for ships of any size once a turn every turn.
Generating net incomes in excess of 1tc wasn't massively hard either, and in my last game, played on the largest available galaxy size, I had generated a net income of 5.54tc on income of 6.09tc. I had reached the money-cap of 999.62tc with plenty of time to spare, and quite literally had, and even was making per turn, more money than I could possibly spend.
10) Technology
Three main gripes here.
Firstly, the lack of control over how to spend my research points I found especially frustrating. In particular when I played a game out to finish the tech tree, I was faced with a situation of having to still spend 10-15% of my research efforts on technology trees I had already completed.
Secondly, the rate of technological progress itself should be scalable. I would like to be able to play on a very small galaxy and still complete the technology tree. Likewise, I'd like to be able to play on a very large galaxy and slow technology progress down sufficiently to make completing the technology tree a more sedate affair.
Finally, Terraforming. I've almost got a love/hate relationship with this. On the one hand, I couldn't imagine a 4x (space) game without it. On the other, the way Horizon implements it makes it the "I Win" button of the technology tree I believe. After a game or two, I learned to focus all research on this advancement, since it's discovery would allow the number of planets (and their populations) I could build up to rocket skywards (with all the benefits to my Finances this entailed). Despite the odd protestation from computer players (those I wasn't already at war with) about my "Reckless expansion" the AI really didn't seem to have any answer to this strategy.
Within a dozen turns of discovering Terraforming, I would have vastly more planets than any other race in the game, and be, as the news reports would affirm, swimming, nay, almost drowning, in credits.
I think the Terraforming system needs to be completely rethought. I would suggest, at the hazard of making Horizon even more similar to MOO than it already is, borrowing from that legendary game once again. Implement a system of terraforming, available much earlier in the game, but with more gradual advances, so that the concept of a "race to Terraforming, I win" strategy is not quite the game breaker it is.
By allowing Terraforming much earlier, but with severe initial production and trade benefits, and having those penalties only slowly mitigated by further terraforming advances, I think would make for a system that is more enjoyable, and one the computer players would be more likely to participate in (in all my games I didn't once see a computer player achieve Terraforming).
11) Game Options
Again the lack of "Random" options for quantities of stars, planets etc in the Game Setup menu is a big miss. I'd also say the lack of a "None" option for Special Planets is odd given it is available for Qty of Anomalies. A "Density" option would be nice too, allowing for a medium sized quantity of stars to be packed into a small galaxy and vica versa.
An option for Galaxy shape, as commonly offered in other 4x space games is, I think, a needed addition here, as it makes for interesting variations in grand strategy (think spiral arms, elliptical, chaotic and other types of galaxy shapes).
Finally, the lack of a technology progression slider as mentioned in the previous section limits the games potential diversity.
Since the replayability of a 4x space game is heavily dependent on how much flexibility you have in the setup of each individual game, this is an area I would especially concentrate on improvement.
Overall Summary
Well, there you have it. I hope you found this feedback useful. As I mentioned before, despite concentrating quite heavily on negatives, I've found Horizon quite enjoyable and it'll take a place in my cabinet of 4x space games with ease.
I look forward to seeing this game progress further, and would be happy to answer any questions about my review that anyone may have.
Regards to all.