First off, I don't think Donny is missing a tooth, I think it's a gap between his teeth. I originally didn't like it, but then it reminded me of Double-D from "Ed, Edd and Eddy" which made me like it more since Double-D was the Donatello of that show.
As for Casey, I never cared for him too much. He was a lovable lunk in the 2003 series, but in all the others he's just a hot-head, testosternone case.
Also, if there really was no hope, I doubt the writers would have been building up this relationship from the very first episode.