I love me some mindless games. Serious Sam HD, Painkiller, Doom, Just Cause, Dead Rising. The list goes on, I can play these games for ages just enjoying the simple gameplay even with its flaws. What bothers me is a mindless game with the pretense of being more.
On the flip side I hate it when a mindlessly fun game tries to act like it's more than that or just blatantly tries to snuggle up with the big shooters of the day, I'm looking at you Duke Nukem Forever, yes YOU with all your wannabe COD/Halo gimmicks and your stupid cutscene narrative, 2 weapons at a time, regen health grey/brown slow as hell movement stuff!
Oh some non FPS titles to add now. Streets of Rage 2 (especially on co-op) Gunstar Heroes, Saints Row 2 and to some extent Super Meat Boy.