Former major leaguer Huck Flener lost an eye after being struck in the face with a batted ball. Not a crushed skull, but this story is not for the squeamish.
http://www.usatoday.com/sports/bbw/2001-02-07/2001-02-07-majors.htm
And of course, Ray Chapman was killed by a pitched ball.
The worst I personally witnessed (live on TV) was John Ellis breaking his leg sliding into second base in the mid 70’s. I’m not sure if he was still with the Indians or had been traded to Texas and they were playing the Indians – it was every bit as nausea inducing as Joe Theisman’s leg snap. I can still see it absolutely clearly in my mind – him sliding over the bag and catching his foot as the rest of him just kept going. I also remember having a game of my own the next day and coming up about a yard short of 2nd on a slide after a double - completely unconsciously.
LA Dodgers catcher Steve Yeager was impaled in the throat when a piece of a broken bat pierced his esophagus in 1976.
I’d say that Buster Posey’s leg fracture last year was pretty bad. Broken bones and torn ligaments. Ouch.
Dave Dravecky’s arm snapping in 1989 was pretty nasty == sad too after he’d come back from cancer.
My dad tore the ligaments in his ankle sliding into home plate in a softball game.
Sliding is dangerous stuff.
Mike Coolbaugh was first base coach for the Tulsa Drillers in 2007 when he was killed by a line drive that hit him in the neck. Tragic; I saw him play many times for the Memphis Redbirds, back in the early '00s.