3) Like every NBA player...wait. Like every good athlete, including Dean Cain, Shaquille O'Neal cannot act. The exception is Sean Connery, of course...but his League of Extraordinary Gentlemen movie equally sucked when compared to the comics version.
Bah! There's lots of good athletes who can act -- the dancers and acrobats, martial artists, etc. Heck, Superman's initial build was based off of Douglas Fairbanks.
But, if you want to limit this to traditional pro sports, Mark Harmon's done a number of good acting stints on TV. He was in the running to play Captain Marvel on the '70s TV show. Bernie Casey's a decent actor and Pro Bowler.
I'd totally agree with you if you were just talking NBA players.