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
Very true. I used to belong to the same school of thought and didn't ever touch a console FPS. Halo proved to be very influential for the FPS genre, not only in terms of controls, but also with (semi-)innovations like well-designed vehicular combat, and restrictions to the amount of weapons one could carry around - making picking up new weapons a tough, strategic choice.
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.
I'm curious, do you prefer H2 and H3 over the original because of singleplayer or multiplayer aspects? Or both? I for one think the original Halo: Combat Evolved is still unmatched in the franchise in terms of the multiplayer mechanics, skill-requirements and balancing.
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.
I agree that the original Halo had some repetitive level designs - but it also had some of the
best level geometry I have ever played in.
Silent Cartographer remains one of my favorite singleplayer and co-op environments to this day. It wasn't even that big a level, but it
felt huge, and it gave you several routes and objectives to complete in whatever order you saw fit.
Halo 2 and 3 took away some of the more sandbox-y elements in the level design, they heavily restricted exploration via invisible barriers (H3 is especially guilty of this, H2 not so much) and they prohibited backtracking through the levels by cutting you off at certain points (mostly a hardware-related issue).
I would have hoped for more non-linear/semi-linear level designs in H2 and H3 - in short, more
Silent Cartographer 
Reach looks promising in that area as well.