I say this as a hardcore fan of the franchise but my patience is wearing thin as the game's technical problems are slowly eroding any fun I got out of this game. The game is constantly crashing about every 1/2 an hour on average for me. The game might be playable in 4 - 6 months time but for now, it is definitely not worth playing.
Game mechanics are more or less classic AC. That's great if you enjoyed any of the previous instalments. However, because Ubisoft has decided to force Multiplayer on us. Here's what you are missing out on if you just want to play the game totally solo single-player and not participate in any co-op or any of the guild (called clubs ingame) content (which I must add, the club competitions are NOT EVEN RELEASED YET) You are completely locked out of the 5 star gear upgrades if you are a solo player meaning you will hit a glass ceiling in terms of gear.
The Armor set in Unity that can be unlocked is basically a reskinned Altair's costume (Either a Pro or Con depending on your preference), however it is basically a skin and has no statistical advantages so it is pointless to pursue if you are not a fan of the looks. I should also mention that you need to collect about 12 seals to unlock the gear which means it will take a while to do as well.
The Parkour is great in this game and probably the best in the entire series so far however as usual, it has a lot of stickiness issues. I've basically bounced about on all 4 sides of a window for 30 seconds or so because the game just doesn't want to let me climb to the exact spot to climb into a building.
Level design is much better than the last few games. Now they give you a more open area and let you figure out which route you want to take to get your target which is pretty fun when you perfectly execute an assassination. However there are a few levels (or some spots used in assassin missions) where the layout and guard density can get pretty ridiculous. Overall though the level design is pretty good in Unity.
The Stealth detection mechanics are pretty spotty in this game. I have encountered issues where guards were alerted of my presence after I stealth killed another guard on another floor or issues where guards all across the restricted zone are alerted to me when I killed a guard and there were no other guards looking my way.
Combat is a lot more difficult compared to previous games (which again could be a pro or con depending on your preferences). No longer can you fight off 20 or so guards and win easily. All guards carry guns which can take out 1/2 to 3/4 of your HP if you stick around too long so you have to try not to stay in combat for too long. The Double Assassination mechanics seem to be pretty spotty too in my experience. Even with 2 enemies clearly highlighted for a double air assassination, I have found myself frequently seeing Arno only hit one target which causes me to aggro the 2nd guard and consequently every guard in the zone is alerted to me.
The Story is pretty meh and doesn't really take advantage of the interesting setting of the French Revolution which is pretty unfortunate. You only seem to read about the events of the French Revolution behind the scenes through the ingame journals or dialogue but are not really involved in many of the big events. I can't think of a single character outside of Elise who is memorable or even has any kind of personality. Arno is okay but is a poor man's version of Ezio.
Ubisoft has also included chests that rely on you playing a Mobile App to be able to unlock. I decided to give it a try but uninstalled it 2 days after playing it after it reset my progress twice. Another broken component of ACU... There is also an AC Initiates web game which I'm not too sure what the point of it is as it is still half built at this point on the website and it has also reset my level earlier. To unlock the legacy skins like Altair's Armor, Ezio's armor, etc you need a certain Initiate level before you can open the chest. (Seeing a pattern with Club Competitions, Companion App and Initiates? I'm not sure why Ubisoft is adding in all these extra things to the main game when they are all pretty much incomplete, unfinished and bug-ridden)
Frankly if Ubisoft couldn't get all this working, they should either have left them out of the game completely and made sure their main game is fully featured and actually works properly or delayed the game release.