I'll elaborate. I agree with both rooster and Vindictus on their points, but will also add some of my own.
The entirety of the gameplay is shallow and boring. The combat is clunky and feels very unnatural. That's my biggest issue with the series other than the really pretentious and unnecessary story elements. Why are we playing as a modern day guy accessing genetic memories? Why can't we just play as the Ezio or whatever his name is? Was this really necessary? It's like they couldn't figure out how to make the story work without adding a little twist to it that feels tacked on and unnecessary when compared to the gameplay. Plus, the story feels like it was written by a mentally handicapped Dan Brown. This is Ubisoft, after all, so I'm not really surprised.
Interrogation is entirely unrealistic, too easy, and predictable. You follow a guy around until there are no guards. Then you knock him around a bit and he tells you everything. This happens every time. No variation. It's not fun. This also applies to things like eavesdropping (sit on a bench and press x, ohhhh so cool). Then there's the retarted flag fetch quest where you have to collect 40 or so flags just to get information from this one guy. What a tedious waste of time.
The game is based on stealthily killing people, yet the game makes it very hard to do that. In theory all you have to do is sneak around your target until the guards are gone, but there are far too many guards and I almost always ended up alerting them which resulted in a street brawl between the guards and if I got lucky the target who, for some stupid reason, thought it was a good idea to join the street brawl with an obvious assassin. Sure, I still kill the target and a bunch of guards, but that's not assassination. It doesn't help that the controls are unintuitive and clunky, making the combat annoying so I just end up doing the same basic moves over and over to dispatch all my enemies.
Plus, if you're trying to find a target, good luck spotting him in the vast clusterfuck that are the various streets in the game. Add the weird cyberpunk-esque lines and data floating about above people and it becomes very confusing very quickly, not to mention out of place and weird for the setting.
The world itself is pretty cool, though. It's large enough and diverse enough to be interesting, but unfortunately the lackluster gameplay kinda ruins that for me.
The parkour elements are also cool, but are boring after about 30 minutes. All you have to do is run up to stuff and press a single button to do every acrobatic feat ever imaginable. There should be more variation with the controls when it comes to free-running.