There are two things I look for in a center:
1) Can he step in and play 30mpg if Jermaine O'Neal is injured for a month-long stretch?
2) If he and Glen Davis were both on the roster, would Doc be willing to play him instead of Big Baby at the end of the fourth quarter in a close game?
Height, length, rebounding skill, inside scoring, etc. matter, but only so far as they contribute to a potential free agent being a yes to those two questions. I'm willing to give in a bit on #2 if Jeff Green can mature into that role.