Nice thread, - I would say...
Beginner - L1's to L4's (B-A grade average)
Decent - L4's to L7's (B-AA grade average)
Dedicated - L4's to L9's (B-AA mostly with sometimes AAA)
Good - L6's to L10 (' ' ' ' ' ' ' ' ' ' )
Very good - L6's to L11 (mostly all A's/AA's minimum)
Excellent - L10 to L12 (most all HC, FC, or AAA)
Thats a pretty wide scale however..
Sometimes I've seen players who can't clear a single 11, but can AAA 8's very simple. However I consider both players to be good. This is still very much a game... And to call someone good because they can AAA plenty of 8's.. and put them in the same rank as the guy who's getting B's on 12's, - is a little off.
To me, beating IIDX will always be about clearing the hardest songs. The timing comes into play for better score, and quality of clear sure, - but if you can't clear a song.. someone else out there can. And that someone else will always be better. Practicing harder clears also ties into timing, so I think increasing your timing comes naturally as you can clear harder and harder songs and just play more really.
I would say if you can clear 11's, Your above average. If you can clear 10's? Your dedicated to IIDX and a skilled player. If you clear 12's and AAA like 70% at least of the entire franchises tracklist? Then your like the prodigies of the game. | 
<- Anaglyph Iroha!
