Three quarters of players are below masters/diamond, so presumably it's reasonably accessible even below those tiers.
> I've always been able to compete at an average level in most ranked multiplayer games, but it seems with SC/SC2 it's always one end of the spectrum or another.
What do you mean by this? No matter how good you are, unless you're the very best, you're going to get to the point where you win about half your games in SC2, as with any other 1v1 zero-sum match-made game.