While I would not call Theo the best center in even the Eastern Conference (Mournings back, Wallace is a monster), I think he would probably rank in the top ten when healthy.
Offense
I am hoping he can get back to averaging double figures as he did when with Philly. He shoots a decent percent (Career 50%). Maybe he needs a few more plays run his way.
Defense
I think you are off base on the defense comment. The guy lead the lead in blocks.
Boards
While some might say that GRob impacted his boards last year, I would love to see him get more boards.
Here's what Mike Khan had to say about him as he ranked him the 8th center in the nba:
"8. Theo Ratliff, Atlanta Hawks: Before breaking his wrist in Philadelphia, getting traded to the Hawks, and injuring his hip and abdomen, he was one of the top young big men in the game. Now, he's still a good defender/shot-blocker, but hasn't improved as a rebounder or a scorer. He should be stronger this year. "
No he isn't perfect, but he could start on most teams in the NBA including many of last years playoff teams.