He's not a super highly touted guy. He's done VERY well this year. High OBP, some pop and some speed. For a MI, that's pretty good, but I'll let Sickels tell you:
Semien is a 6-0, 190 pound right-handed hitter, born September 17, 1990. He's a good overall athlete with no weak tools, although he doesn't really have any spectacular ones, either. He's developed more power than anyone but the White Sox expected, enhanced by his superior feel for the strike zone. His running speed is a tick above average but he makes the most of it, evolving into an efficient and effective stealer (20-for-25 this year), which again is more than anyone but the White Sox thought he could do before he was drafted.
The Sox used him at all the infield positions (except first base) at Birmingham. The consensus is that he's best at second, but his arm is strong enough for third base. His range is marginal for shortstop, although that's his position so far in his first few games for Charlotte.
You have to be impressed with what Semien has done over the last year and a half; if he has a good August in Triple-A, it will become increasingly difficult for analysts to ignore his performance. If he continues to max out his skills, he could be a regular. Even if he's just a .240-.250 hitter at the major league level, his ability to draw some walks, provide some power and speed, and play multiple positions should make him an attractive asset in this era of short benches.