Just finished it, and I guess this would be only the second Assassin's Creed game I've finished. Was bored to death with the first, but finally finished it because I was wanting to play the series in order, greatly enjoyed most of AC2, then just got burnt out three-fourths of the way through and haven't looked back.
1. This is nothing but Assassin's Creed with Batman combat.
2. The combat is as easy as Batman, yet much more visceral and the sound effect with the wraith stun flurry attacks is pretty cool sounding. The only time it becomes "difficult" is when the alarm gets sounded. You're pretty much forced to run away and start again. Or at least find a high place to hide for a minute or two until things calm down.
3. The environment, while not particularly "ugly", looks terribly bland from both geometry and aesthetics. There's practically no variety. It's all browns and greens with some green until the second half, then it's all browns and greys with a lot of green. Assassin's Creed beats this game pretty badly with environmental and architectural detail. Batman completely demolishes this game in aesthetics and atmosphere.
4. The story is "serviceable" at best, with not a single character to actually care about. Assassin's Creed 2 had a way better narrative and the Batman games own both in regards to interesting characters.
5. Main quest line is extremely short (sub 6 hours short unless you're just really bad and die a lot). The majority of the game time revolves around collection garbage or Outcast missions that serve no purpose other than to give you money to buy weapon upgrades. I finished the game with 80% of the missions completed, all powers and upgrades and only completed 8/20 Outcast 4/10 "Survival", 4/10 "hunting". I saw no point at all in doing those. It's just a bunch of repetitive busywork that all games of this type suffer and seem to think it's meaningful gameplay when it's not. It's just garbage padding.
6. The weapon missions were just challenge maps. You don't actually get any reward for doing 10/10 sword, dagger, bow. Well, I guess if a skin job is a reward for you, then there's that.
7. Last fight was the most anticlimactic ever.
8. I saw zero purpose in doing any of the red icon missions after the final tier of upgrades was unlocked. Yet, plenty still exist after to reward you with Power that doesn't do anything.