Halo's excellent
I guess some people don't know a good FPS when it hits them on the head.
Halo is a very good game, the best in its generation. It makes PC most PC FPS (including HL2) feel like, well, toys. Why? Because it has an incredible sense of realism. When you turn, it's not instant. You don't glide along the ground as you run. You don't have any really crazy weapons. They even included the UI configuration as part of the game experience itself.
The AI in the game is also good. At the highest difficulty level this really comes into its own - not only are the enemies crack shots but they're also bright enough to dive for cover when you've got them under heavy fire.
It's also well balanced - all of the weapons have a use, and the effectiveness changes depending on the difficulty level. Most of the levels (a certain latter one excluded) are also really well designed.
Oh, and unlike some recent console FPS (Bioshock), you have some choice over control schemes - you're not forced to try and play with a controller setup which is impossible for you, just because the developers are too lazy to implement an options screen.
What Halo did was perfected the whole experience and balance. Non of the innovations (clever-seeming enemies, shield-system, realistic controls, excellent balance) are so immediately obvious. They're so right that it feels like they were always there in FPSes.
Before playing Halo I was a massive Half Life fan. I was of the opinion that you couldn't do a shooter on a console. Halo changed that, to the point of reversing my opinion - in the most common configuration the PC really isn't the best format for consoles.(kb/mouse kill atmosphere, and most people don't have their PC in their home cinema system).