You can! Just pick your region first, ie. US. Then pick your battlegroup.
Why don't you show specific specs?
Since all players have dual spec there's no way to determine which spec they use for arena, for a particular comp, etc. unless of course both of their specs are the same tree. (Which I'd imagine is not most.) So rather than guess or pretend to know people's specs, I choose to leave it out.
Whats with the red/green %'s?
It's an attempt at data normalization in reference to the % of people playing a particular class (who arena) in general, vs those playing that class at a given rating. Basically, anything higher than 0% means theres too many of them, below 0% means not enough (ideally) ;p