It is not a midfielder that is needed, if anything the midfield is the only strength. When Donavon decides to actually play soccer he can be a dangerous threat. If Beasely learns how to dribble a soccer ball / play defense he could help. And if Dempsey chooses to take risks at the right moments instead of every moment he can be a risk. With Bradley and all the other young guys, the midfield is a good if not great one.
There is not a bona fide american striker. At all. If Ching get put into a world cup game, may as well hope for a 0-0 tie. He is a liability, at best. After that, theres Altadore, but he's not come into his own yet. Eddie Johnson was a flash in the pan. And noone else makes you think 'give this guy the ball at the top of the 18 and it's a goal'. just the threat of a goalscorer will open up the field for the rest of the team, forcing opposing defenses to pinch in a bit, leaving the wings open.
If anything, though, the true weakeness is the defense. Hejduk is awful, just awful. No way he should be a national team player. Onyewu is a big goon with speed. He can't cover the best in the world, he can't even slow them down. His boneheaded play cost the US the ghana game (yes it was a penalty, yes it was the correct call, yes he shouldn't have done it) After that there is no names that would strike fear into opposing strikers.
In my opinion there are two pieces to the puzzle missing. A shutdown defender to play next to Onyewu in the flatback, which will minimize the damage the flank backers create. And most importantly a true striker. Defense wins championships, and the best defense is a great offense. Control the clock, control the game, control the score.