Rico Fata. The first guy I thought of. In fact on his draft day when he was drafted 6th or 7th overall with the Flames it was posted on the jumbotron the he had "limited hockey sense." Not very classy, but it was kind of true. Fata was the kind of guy that could probably skate just as fast as anyone in the NHL, even today, but his head never caught up to his feet. He would get the puck caught in his feet all the time. In the '99 WJC he was flying all over creation like a madman but never capitalized on anything really.
There have been a few guys like this. Ken Yaremchuk was already mentioned, and Dave Archibald was another very high pick who had great wheels but no head for the game at all. Tom Chorske was similar, too - had size, wheels, and a huge shot, but no hockey sense, but at least Chorske managed to carve out a solid career as a checker.
__________
For me, though, the definitive guy along these lines was Chris Joseph. As a defender, the guy had everything you needed to be a perennial all-star and top-10 defender in this league. Was 6'3, 210 and had a smooth, effortless skating stride with excellent lateral mobility. Wasn't afraid to play physical. Could throw crisp outlet passes 80 feet tape-to-tape with the best in the game. And had a great shot, both a quality slapper and a laserbeam of a wrister that could beat a goalie clean from 30-40 feet out. Was a 5th overall pick, was playing in the NHL a few weeks after his 18th birthday, and a cornerstone of the Paul Coffey trade while still a teenager. Was supposed to be a franchise defender, and had all the tools be a star.
But he was just dumb as a fencepost in his own zone. Hung around the NHL for almost 15 years as a depth defender, and could never learn how to read the play, and could never stop himself from chasing the puck around like a chicken with its head cut off. As a result, he was never more than a depth guy. I've never seen a player so easily and so consistently confused by a forward crossover on a 2-on-2 rush. Simply couldn't maintain his side of the ice. Huge waste of talent.