I'm well-suited to the core gameplay concept of Assassin's Creed: it's a sandbox where you get to use major historical city, in this case primarily Florence and Venice, as your own parkour playground. I'm not actually so keen on the Assassin part, but there's enough fun running, climbing, racing, and exploring to keep me interested. Speed and stealth are both fairly effective ways to avoid combat and in a pinch you can always beat someone up with your fists, although it's fairly tedious.
The game wasn't particularly hard. I never really got frustrated although as a downside I didn't always feel challenged. I've yet to finish the puzzle-oriented 'truth' unlocking session but I'm likely to soon enough. Similarly, one of the biggest delights of the series is climbing up to high points and looking around and I've got about a half dozen viewpoints left to find. I can live with games that are this forgiving; I think the key is trying to play with style rather than simply achieving your objectives. The dialog feedback from passersby and the notoriety system both do a reasonable job of docking you if you get too sloppy.
Assassin's Creed probably won't make my all time top 10, but I think the gameplay in many ways takes some of the mechanics of Thief while making them a lot more fun. I'd rather play a Thief than an Assassin, but I'd take Assassin's Creed's jumping and climbing over the jumping puzzles in Thief any day of the week. Also, amusingly, I think the death toll in Assassin's Creed is still a fair amount lower than in Uncharted despite the latter being about a fortune hunter and not nominally a killer by trade.
If I could add one thing to the series, it might be to provide a bit of parity between the deaths of assassin targets and the nameless guards that you may kill on the way. The targets all get a benediction, specifically set up as the way your character shows respect for the dead. However, no such privilege is extended to the average guard, who presumably is actually a better person than most of your targets. There's already an option to loot bodies; I don't think showing respect for them would be all that hard and it would decrease the disconnect between the gameplay, characterization, and story.
Source: Loaned to me by my friend Ryan, thanks Ryan!