Most of the animosity towards Halo and COD boils down to the fact that some may simply not recognize that you don't have to think to have fun.. Anyways. Halo and COD and Half-Life(or Tf2/Portal) are vastly different in their play styles.
Halo, for instance, is fast paced hihg-octane action that requires split second decisions on the go. That doesn't make it "mindless" so much as it leaves you with little time to do some serious thinking as to the results of your actions. This might be a game synonymous with Goldeneye for Nintendo 64, if anyone has forgotten.
COD on the other hand, is just the same as Halo in the respects of fast paced action, but there are more tactical and strategic options available to you in the game. Also, the pace is not as fast as Halo, and leaves you with a bit more time to think, this is also supported by the fact that you have items or enhancements that augment the game play and allow you to worry more about whats in front of you than having to THINK about it for ten seconds and end up possibly dead.
Half-Life, and Portal's pace are set slower. Half-Life is indeed a thinkers game, but not on such a level as portal. In Half-life, the object is to use your wits to solve problems ranging from a room filled with foes to navigating deadly toxic wastelands. This means the pace can vary from a halo level, to a Portal level. But regardless of pace, there's always some way that you can manage to catch a breather in the thick of it. Portal, on the other hand, requires nothing but thinking, and even at that, it takes no rocket scientists to move a cube onto a GIANT RED BUTTON on the floor. And while the puzzles do get trickier as the game flows on, its not as ive youre straining yourself or bursting a vein to figure out a puzzle. Almost all puzzles in portal can be solved easily and with little difficulty, but that's not to say simply "not thinking" works. Portal is certainly still a game where you must look before you leap.
TF2 is somewhat similar from COD or Halo in it's pacing. The class based system means 9 different play styles (to the exception of unlockable items, maybe add about 18 styles of play) Thus the pace is set by the class that you have chosen. Pace in Tf2 can also be set by the map played. Where dustbowl may be one big pushfest, Twofort can drag on like a slug on molasses.
My point? Ease of Play != Skill/thinking