Size for one. You can't really be 6'10" and be considered a true center IMO. The other of course being you actually play at the 5.
WHAT? Jeeze dude, stick to talking about guards or something cause you don't seem to know shyte about the 5.
Hakeem, Mourning, Moses, Wallace, and Russell, all 6'10 or shorter who were true centers.
Tim Duncan technically is a center but he doesn't ever play the position. Pau Gasol would be a center as well but he doesn't play that position either.
Centers traditionally play and defend in the paint. TD is a center, plain and simple. Gasol is more of a PF but is also a "light" center. Guys are just so much bigger at other positions, especially at the PF, that a traditional center like Gasol becomes a PF/C tweener. Other PF/C tweeners IMO are guys like Memet Okur, JO, and Wallace. They would definately be strait C's about a decade or two ago but with the new "mobile" big man, it seems the lines are getting blurred.