Well to me Assassin's creed wasn't that great, but I think AC2 will be amazing, because the first one felt like a game that warranted a sequel. A bit like a beta or a rough build of an engine. The running mechanics are great, the fighting mechanics work well, though counter has no balance (hopefully they fix that) the graphics, animations, character actions, and AI is good, though a bit predictable. Really the only problem of it is lack of variety that leads to repetition, and since they already had the build all their effort went in to fixing that problem.
From what I've heard there's like 30 different weapons they've added, along with added more variety to hand to hand and hidden blade combos and more hidden blade add ons like the needle and the gun, giving more variety in killing methods
There's now 3 types of standard enemies, one that's good against combat, one that's hard to outrun, and one that's hard to hide from, meaning that combat not only gets more variety, it makes it so you actually have to asses the situation not just run in mashing RT and x.
You can now pickpocket anyone and there's a currency system to buy upgrades and unlockables, which gives you something to work for.
There's much more variety in side missions, which gives you a lot more to do.
Assassin's Creed's only two (major) flaws in my opinion where that it was repetitive and combat was too easy, and now that it appears like they've assessed and fixed them. I'm expecting AC2 to be one of the best games ever. Granted it probably won't be, but if they do everything they say they've been doing and do it right, it could be well within the realms of possibility.
Plus, AC is something original, and I give a lot of credit to that. I would much rather play something that feels fresh than some generic War or Zombie FPS.