He is a poor man's Rondo. Seriously. He's a good passer (but not quite as good as Rondo in terms of assists per 36 or assist/turnover ratio) and a good rebounder for a point guard (but, again, not quite as good as Rondo in terms of rebounds per 36). He's an above average defender; he is better at getting steals than Rondo, but not as solid in his rotations (and fouls a bit too much).
In terms of scoring ability, Rubio is what all of the Rondo-haters THINK Rondo is like. I mean, everyone gives Rondo crap for not being a good scorer, but when you actually look at his shooting numbers he is great. Not a 3 point threat, and puzzingly bad from the line, but overall an above average scorer. Rubio, however, is coming off of what is arguably the worst shooting season for an NBA starter ever. He is atrocious. His effective field goal percentage is 38%!
So, basically, Rubio is just the mediocre version of Rondo.