People dont like Modern Warfare 2 because its broken. Not in the way of glitches, but gameplay. Its not balanced, at all.
The way the game balances itself out is by making a lot of weapons and combinations over powered. Not the mention the perks and Killstreaks. The entire game praises luck and camping over natural skill. AC130, attack choppers, Nuke, Commando Pro, etc... This is made worse by the fact that the weapons drop you so quickly, you cant really counter a person who hides and camps.
I like the multiplayer of Halo 3 far better then MW2. MW1 wasnt too bad (as it was fairly balanced), but Halo 3's just seems better. The fact that you can all absorb more damage means that it takes away some of the appeal of camping. Not only that, but the players start with even weapons, meaning the focus is shifted to teams controlling an area of the map, as opposed to "were all rambo". Granted, you still get idiots who play that way, but thats their problem.
Modern Warfare 2 just stinks of bad mechanics. The only reason I own it was because it was a gift; the only time I put it in the disk tray is if Im asked to, and I usually troll when I play. For all my biting words, Riot Shield + Rocket Launcher Combo is still a fun way to troll.