Its too early to throw Marat in here, though there really is almost no excuse for him not having won at least three slams by now, buy I digress...
Mark P, if he never wins a slam, IMO will go down as a massive underachiever. Even with the injuries.
Kafelnikov was a good one - he should IMO get into the hall of fame (one of only a few guys to have won 25 doubles and 25 singles titles, a stint at number one, consistent top tenner over a decade in singles and doubles, multiple slam winner in dubs and singles, etc etc), but could have done better potentially.
Just a thought, but has anyone noticed that a lot of the really really talented players who never seem to quite max out their potential are among the most intelligent?
Rios for example, may be an ass, but he strikes me as being extremely bright. Both he and Kafel seem bored half the time they are on court, like it just isn't stimulating enough for them.
I am not saying you have to be dumb to be a consistently great player, but it is interesting in my opinion as it appears to be a pattern among *some* of the underachievers.