Halo doesn't have the same impact when you play it on the PC because part of the appeal of Halo CE was that it was an FPS that played extremely well on a console, something unusual at the time. I used to be a PC purist, whenever I played shooters on consoles I always got frustrated by not having a keyboard and mouse, but when I first played Halo CE, I didn't have the same frustration. Halo really helped console shooters and drove them in the right direction, and now I prefer playing action games on consoles and only use the PC for strategy games.
I enjoyed Halo 2 and 3, I prefer them both to the first game, but they didn't really innovate as much as the first game, they threw in some new weapons and abilities, but the core game remained the same. Also, they took away the pistol of doom.
Sometimes I could take down a hunter with one shot on legendary difficulty using that pistol.
My brother is a Halo fanboy and he's really psyched for Reach, so we've been playing through the the games over the last few months and I have to admit that Halo CE has some of the most boring level designs in a video game. Take that mission in the ice canyon where you walk through was is essentially the same room about two dozen times, it's really repetitive. Then, later in the game, you have to walk backwards through that same level! And don't even get me started on the library level.
The best thing about Halo 2 and 3 is that they shook things up and you rarely feel like you're walking through through the same room twice.