I don't know why AC1 gets so much hate.
It's all about the gameplay - which is severely lacking in AC1.
I'll get to the explanation...
The combat was a bit more difficult, but only because it was more complex. The game actually played more like an assassin/stealth game compared with the Michael Bay action games that followed.
B/c the side missions were repetitive; side missions lacked variety; and the NPC character models and their voice-acting were often cut-and-paste; and gameplay variety was lacking.
And you were FORCED to do these repetitive side-missions before you could even embark on main quest missions - something AC2 fixed!
Killing your target was a thoughtful process. You had to plan well and execute that plan properly to achieve success. After that you were rewarded with interesting/thoughtful dialogue.
Main missions were excellent....
...but, the last 1/3rd of AC1 turns into an action-fest and becomes a different game entirely.
Following AC1 you are more akin to a homicidal maniac just tearing through the area looking to sate your bloodlust.
AC2 gave the player more variety w/ a lot more missions types.
The story was actually done correctly as well. In AC1 the memories merely supported the main story of the current assassin/templar struggle(Desmond's time). The following games merely tacked on Desmond portions for continuity sake and made the memories the new focus.
AC1's story was pretty good....
...until the cliffhanger. Usually, I'm not fond of the AC cliffhangers.
ACB had one of the more jaw-dropping ones.
Sure, it was far from perfect, but I think it gets a bad rap as one of the worst of series. It should be, at worst, right behind AC2 and probably would edge it out in my opinion.
Everybody's got different opinions - but I think AC1 was very disappointing and an very overhyped.