It's only a 12-character roster because BlazBlue is a brand-new, 2D fighting game. It isn't very common nowadays for most fighting games to have such small rosters because almost every single fighting game coming out these days is from a long-running series (SF4, Tekken 6, Soul Calibur 4, DoA4, Virtua Fighter 5, etc) and are usually using 3D models (which are far easier to deal with than drawing thousands and thousands of hi-res, 2D frames).
Also, if you look at any fighting game when it was brand-new (not counting cross overs which usually recycled a lot of old sprites), their rosters were relative small (around the same as BlazBlue).
I'm actually not too sure about the balance, after seeing characters like Arakune, V-13, and Rachel in tournament level action. All 3 seem highly abuse-able to the point of having no chance to win (esp. with the first two).
Bad match-ups usually seem pretty one-sided as well, like Ragna VS Tager, 9 out of 10 times, I always see Tager win (and usually by a landslide).
Versing Arakune seems to usually be a huge game of "dodge the homing curse clouds" (which mark opponents even if they block the cloud) and "catch the fleeing Arakune".
Everyone else seems to be pretty well balanced (though I still have a few tiny qualms with Ragna's pathetically low Life count. Yes, I know he drains life, but with his Life count, anything he drains is hacked off ten times over in one or two hits).