There is no reason why at the very least he should have become SWP or a Ji Sung Park. Next to Donovan he had the most natural ability that I ever saw from a US player. At the U-19 world cup, I saw a bunch of matches in Canada. Kun Aguero was the best player there, and Freddy was the 2nd best however Adu was by far the most exciting. About 3-4 times a match you'd hear the crowd collectively go OHHHHHH. He could beat anyone on the wing, score from the wing and his crosses were sick.
Unfortunately in his own words he always fancied himself to be the next Kaka (he said this at 21 hoofing it in god knows where then - it wasn't some pie in the sky comment by a 15-16 year old) and wanted to play attacking mid. He was absolutely terrible at attacking mid (DMs and CBs would easily just gang up on him and nudge the little man aside easily + while his crosses were great his through balls in the midfield were a waste of everybody's time -- Coleman is quality but imagine Coleman as an attacking mid, that's Adu) and wouldn't listen to coaching. If placed out on the wing, he'd just cut inside, get dispossessed easily and leave a nice 30-40 yards of wide open space for opponents to counter with.
The US was good enough to win that U-19 WC but they lost to Austria, why? Well after setting up Altidore for the 1st goal after beating a man and putting in a cross to Jozy for the goal; Freddy decided that he wasn't going to play wing anymore and set up shop in the midfield. It was a disaster, Austria scored one to tie it up and with Freddy running the midfield got another in the OT pretty much by letting Adu hand them the ball and working the counter down the space where Adu should have been.
Sadly he never learned either. When called up for the USMNT, you'd see flashes that he should be penciled in as a starter but after a few minutes into the match he reverts to his own ways. Donovan didn't live up to his potential early because for a while he hated football. Adu IMO is just uncoachable. Its either that or he was in his 20s when he was 13.