Huh. Gaming mediocrity. Good question.
The Halo series comes to mind. It's competently executed, it's pretty enough to look at, but the actual shooting just feels rote. It's all perfectly stuck together and it all works very well, but it's turned into such a masturbatory subculture I just can't work up any kind of enthusiasm.
The same with Gears of War. We can credit it for the now standard cover-based mechanics, but what makes the whole package mediocre for me is the contradictory mechanics and design choices. The game favors protracted firefights, and yet you still look like a 'roided up supersoldier? Dafuq?
Most of the Need for Speed games of the last decade are also mediocre. They seem to be perpetually stuck in "The Fast and the Furious" mode, and everything just seems like they're trying insanely hard to ooze personality - but nothing really comes through. The only positive comments I could think of would concern how the NFS series kept FMV sequences going in an era where everything that was not part of the actual gameplay was usually relegated to pre-rendered cutscenes.
You could also add sports games to the mix. Pick any license and switch the roster around year after year after year, with minute changes and innovations thrown in here and there. It all very much feels like a paint-by-numbers design job.
Then, and I hate saying this, you've got a lot of what comes out of some smaller European publishers, specifically Kalypso Media. Everything these guys produce ends up being resoundingly bland, or is utterly destroyed by a small army of bugs that render their products unplayable.
Who here remembers Dungeons, for instance? It was billed as a sort of spiritual successor to Dungeon Keeper 2, but the mechanics were not only completely different, but badly thought out and implemented.
Another thing that really gets on my nerves is when publishers figure everything can be remade as a shooter. Syndicate is a sufficiently clear indicator that the public mindset isn't quite as consumed with the desire to shoot down mooks as investors and project pitchers seem to think.