For me my main three are Gambill, Phillopousis and Clement. All 3 with Great potential though have not yet fulfilled the promise.
Gambill has all the weapons including one of the most consistent serves yet his Grand Slams have been disappointing. This is similar with Flip (Before the Knee Injury) he has all the weapons the serve well documented a great forehand but yet he has also failed to deliever. Meanwhile Clement final appearance at the Oz Open seems to have hampered him. I think the pressure from the French was too much for him and he has yet to recapture the form he showed to reach that Final. I also think that it is a kind of mental thing as well.
Marat Safin, yes. He is probably the biggest "natural talent" out there.
Nicolas Kiefer is someone I would put there too. A lot of touch and talent, little reflect in his results.
Yevgeny had a lot of good results, but he should have done even better. He is there too.
Those are the biggest ones I guess
Leo7 - he will retire, if Russian wins the Davis Cup this year.
Tennischick - funny, I mentioned that in the Pete and Andre thread the other day. I didn't say he should have won more slams, I just said that he underachieved for so long - bu I totally agree with you.
I think Marat & Federer has a lot of time to remedy their under achieverness (LOL, is there such a word)!
In a word Flip, the guy is 6 foot 6 inches, can whack the ball to kingdom come, strong, excellent service, I guess his heart is not really into tennis. And add to that,his misfortune of being injury prone.