[whswhs]

It's nice if players know the rules, but I don't think it's all that important. I have had exceptionally brilliant players whose rules knowledge was only adequate, and acceptable players who barely knew the rules at all. I still have players who have trouble finding things on their character sheets, but who contribute to games.

Two things that I think are big for "contribute":

They get into character and roleplay the person they've imagined, and do so in such a way that the other players can appreciate what their character is thinking and feeling; they bring them to life in play.

They care about the game being fun for everyone (this is in the article you linked to, and it should be).

I also appreciate players who constantly test themselves by taking on different roles and playing different characters, but that's a bonus.