I don't think he's special, but he's killing the long mid range jumper, and with how good a pick he can set that makes him a pretty valuable offensive player. He's also got the bulk to do damage inside at times.
I think he's going to be a solid pro. You're right in that longevity may be an issue with him, but a number of players who've been huge in college have controlled their weight in the pros (Udanis Haslem, Carlos Boozer, Kendrick Perkins (HS), Al Jefferson (HS) etc..).
I'm not leaping head first as fast as I can to offer Davis a full MLE, but I wouldn't blame some team for doing it either if they think his performance with KG out is legit.
Just think of how good we'd be right now if we had Glen Davis's performance instead of Tyrus Thomas's. We'd be killing the Celtics.