Just taking time out of the day right now to point out, now that Halo is mentioned, that I don't know why the heck that franchise is so popular
.
I haven't played Halo 2 so I can't judge based on that, but I did play Halo: Combat Evolved on the PC (which is essentially the same game, without co-op but with multiplayer support) and I don't know why that game got so many sales, or why it practically carried the Xbox initially, or why it was rated higher on many websites than Half-life. The game sported the worst terrible design I've seen since I started playing first person shooters. It made playing through the game (particularly the infamous Library level) by and large a chore. Not fun. I dunno if Halo 2 is much better or not, but it puzzles me how the franchise took off based on the first game which was really nothing special.
Maybe the comparison is flawed though, maybe there just weren't many good FPS games on consoles when they were rife on the PC.