Instant replay would change the entire dynamic of a game. There's no place in baseball for it. Say a pitcher starts an inning (with nobody up in the bullpen) and it goes like this:
HR on a 0-1 count
4 pitch walk
First pitch single, runners at the corners
First pitch HBP
Now you got the bases loaded after 8 pitches and nobody out... And the manager of the team on defense argues that the HBP call was wrong because it hit the bat... They go review, and he's wrong. It hit the batter and the original call was right... But in the meantime, he just bought his reliever in the bullpen the time he needed to get warmed up and he's ready to come into the game right after the instant replay, rather than the guy on the mound having to face an extra batter or two because the guy in the pen wasn't warmed up.
The pace of the game is right about where it should be... Change that now and you're going to change too much.