Ian Blackwell is one that springs to mind. He has a lot of potential as a hard-hitting batsman, as a foil for the defensive players in the batting side, and bats left-handed. However, he has a pathetic sense of timing and hardly gets bat to ball. IF he could display the same batting ability he does for Somerset, the English batting lineup could be a lot stronger.
Ashish Nehra has a lot of potential as a seam bowler and can be quite powerful in seaming conditions, as seen in the Kingsmead ODI, and can also compete on unhelpful pitches, as in Rawalpindi. However, injuries and lack of urgency have done him no good at all.
Then you have Irfan Pathan and Farveez Maharoof. Both have decent batting ability, but have under-achieved a lot more than England's bits-and-pieces brigade. If they showed a better sense of application and scored more runs, their teams would be in much better shape than they are now.
L Balaji was in very good form, which got him into the team. However, his technique was found out at higher levels- his diagonal run-in and slow pace reduced his chances of getting wickets, especially in BOWLED/LBW.
Some of the aggressive WI batsmen throw their wicket away needlessly, and don't have the best shot-selection and even the dangerous Chris Gayle has been found wanting at times. The same can be said of Yuvraj Singh, along with his struggles against slower bowlers.
By far, the biggest disappointment has to be Shane Bond. He had a meteoric rise in international cricket, particularly in ODI's, and maintained a very impressive bowling average (Tests and ODI's) and put his tearaway pace to good use unlike some fast bowlers of today, but got injured far too often.