Halo is definitely one of those games, it may seem troll-like but it really is, Halo isnt a great game, it isnt a bad game either but it just it isnt anything, it's so incredibly mediocre and incredibly generic.
Lost Odyssey was pretty meh too, the story was good but everything else felt archaic.
Dragon Age: Origins didn't impress me either.
Dante's Inferno...basically a lot of games this gen.
Most of the BIG games this gen arent meh though, most of them are either really bad or really good, the rest we don't pay attention to or they fade out. The only really big game this gen that is meh is Halo, the rest aren't remembered after a year or so.
Can I name two from each genre this gen, let's see.
so Lost Odyssey and Dragon Age for RPG
Dante's Inferno and Devil May Cry 4 for Hack and Slash
Halo and Perfect Dark Zero for FPS
I can't think of any platformers that were really average because most of them have been pretty good or pretty bad.
The new Soul Calibur and Tekken games have been pretty meh.
Gears of War 2 and Lost Planet 2 were both very meh.
GTA IV and Red Dead were kinda meh too. Boring in my eyes anyway. Not bad, not good, just boring.
Not many puzzle games were average.
I dunno, it may look like I've listed a bunch of popular titles to troll but I haven't I genuinely think these games are pretty average, that doesnt mean you can't enjoy them or have fun with them, it just means that they aren't all that great.