Having played through AC2 twice, I went back to AC and I actually love it even more. AC2 was insanely better in terms of gameplay and mechanics, and I love AC2, but the original was truly the best story and I'm a story-nerd. Furthermore, the skills I gained from all that practice in AC2 actually carried back and made the original much more fun.
Specifically, the Savonarola quests were where I was first forced to really master public stealth kills (walk up behind them without attracting attention and STAB). After that, during my second playthrough I practiced those skills and many other skills a lot, and when I went back to AC investigations I gave up on were plenty doable, and even were quite fun to use the skills without the extra ease AC2 added.
For example, Altair can do ledge kills (drop kills) if you set it up right, but it's not built into the engine yet so you have to cheat it by timing a jump and a kill perfectly. Similarly, he can fight with just his hidden blade, he just can't block - but I've gotten so good at the timing that once I get counters I barely use anything else. So much more efficient, and accurate as to what I think Altair would have done. It's not actually hard to travel the entire city without getting attacked once you get good at reading the guards, something I only got after AC2 was more obvious about the guards' stances.
I'll admit the game is very repetitive, but I'm a busy student so it works out fine without bothering me. I don't get long play times, so I never do the same quest on the same day.
Now to be quite truthful, what they really should do is port the story of AC into the engine of AC2. Altair with the smoother free running, accessible ledge and haystack kills, more-logical guard AI, and better support for hidden-blade fighting? Magic.
(I haven't played Brotherhood yet, but I suspect it'll disappoint me. I like the insane difficulty of AC1! My favorite quests in AC2 are entering the Vatican and the Savonarola ones, because they're so much harder than everything else. AC2 made the game so much easier, mostly in a good way, but Brotherhood just overdoes it, I think. Also, I wasn't fond of the monetary system in AC2 - it wasn't well balanced, and seemed like such a half-assed way to add complexity to the game. Overall, it sounds like ACB just continues the trend.)