Footspeed mostly, he's probably the slowest UConn center since Mark Suhr, maybe Carlton. Just watch the substitution patterns. He plays limited minutes against any mobile big with an outside shot, unless Adama has to come out. The moment that guy goes to the bench they sub DC in. Then he comes back out when the other team subs their guy back, and the guys in the chat scream "why are they taking him out". I'm not arguing whether it should be that way, just that it is what the staff has been doing.
When was involved in tennis I was a USTA verifier. The job was to rate a player from 1 to 7 (usually 2.5 to 5+).
When training to be a verifier we watched dozens of marchers over several hours. The players on the court had years playing in adult leagues, and had a consistently solid rating.
After a few hours we all met and compared our ratings. On most courts most verifiers had person "A" as higher than person "B," mentioning footwork, serve, winners vs errors, etc.
A fair amount of the time I was asked why I disagreed, and how I came up with my rating.
"I kept track of the score, and recorded who won and who lost."
My ratings were damn close to a perfect match to each player's rating based on a computer analysis of the several dozen matched they had played.
Ultimately the bottom line is the bottom line. I don't think Clingan has been given enough minutes to draw the same conclusions you state above.