Catch, block, throw and game management say it all. Nowadays a catcher should also be able to hit, but if you're good enough defensively, they'll take any hitting you can provide as a bonus. Two examples. 1st is Dwight Childs, who played part of one summer on our Connie Mack team. He's the Arizona Wildcats catcher now, and Baseball America just described him something along the lines of "Arizona will happily take any offense they can get from their catcher, who happens to be one of the countries top defensive cathers." 2nd example is a friend of the family, who just turned twenty years old. He's from Australia, and was signed by an MLB club when he was 16, solely because of his defensive skills. Baseball America named him as his organizations top defensive catcher in their minor league system last year. He hit about .215-.220 last season. I got a call from him on the 14th, to give the kids his new cell phone number. When I asked what he was doing calling so soon, since he should still be in Australia, he replied "Because I got invited to big league camp, mate."
Neither of these guys got to where they are by their hitting. They're very good guys at catching, blocking, throwing (Childs throwing is incredible)and GAME MANAGEMENT. Both are completely capable of calling their own games, something too few catchers can do well in this day and age of coaches thinking for them.
IF you can do all these defensive things well, you can play for a very long time.