Halo: Combat Evolved was way over-hyped. It's a decent game, but nowhere near as good as people made it out to be. I remember when it came out, I thought it was a big step backwards from Perfect Dark in so many ways. The level designs were generally pretty boring, especially in single player. I didn't like the aesthetics of the level designs--the way everything just felt over-sized and you spent too much time running in a straight line down long, empty corridors. The multi-player maps sacrificed interesting designs to accommodate vehicles. Even the maps that don't support vehicles employed some of the same aesthetics which I found to be off-putting. I generally don't like big open maps with a few random places to hide. If I can just run in a given direction and reach my destination without having to think about the layout of the map, the design is too simplistic. The weapon selection was a bit limited as well. The weapons themselves are good enough, but the variety pales in comparison to Perfect Dark. I also didn't like the regenerative shields. You get shot and just go hide to let your shield regenerate. It kills the epic shootouts where you are just chipping away at each other's health until someone finally goes down.
Gears of War - I also remember this one being very hyped, but like Halo, I found it to be decent, but nothing too special. I found the character designs to be lacking. I just don't find any of the characters in the game to be very likable. The graphics are also very bland and dull with very little color. When I first started playing, I had a hard time instantly distinguishing the good guys from the bad guys, so I was just shooting everyone. The music seemed pretty generic and unmemorable, but that is pretty common in modern games anyway. In terms of gameplay, I didn't really have too many complaints, other than Gears 1 was too easy on Normal difficulty when playing co-op. I never went back to play it single player.