Instant replay is finally here, in all its glorious, expanded capability. This is a good thing for baseball and a long time coming. Home runs were just a start, but now managers will have the ability to challenge one play per game -- plus a second one if the first challenge is successfully overturned. Just don't expect Don Mattingly to be throwing a red flag, however; it will simply be a verbal challenge to the umpiring crew.
The types of plays that can now be challenged:
Fan interference (especially in Yankee Stadium!)
Stadium boundary calls (especially at Tropicana Field!)
Force play ... although a fielder touching second base on a double play will NOT be subject to instant replay
Tag play, including steals and pickoffs
Fair or foul in outfield only
A trapped ball in the outfield
Hit by pitch (this one could still end up being controversial, considering how difficult some are to see, even on instant replay)
Timing play -- such as whether a run scored before the third out was recorded
Touching a base (which also requires an appeal)
Passing a runner
Record-keeping (such as ball-strike count, outs, subs, score -- let's hope the score of a game never has to be challenged)
OK, got that? One interesting note is that the phantom double play -- a.k.a. "the neighborhood play" -- is not being subject to review. Why would that be excluded? It could be an issue of player safety; you don't want a second baseman standing in there too long to make sure his foot is on the bag and then getting steamrolled by Bryce Harper. Also: It's a tough play to call, since so many of those are bang-bang plays, with the ball reaching a player's glove and the foot leaving the bag almost simultaneously.
You can read how the review process works here. As with the NHL replay system, there will be a central Replay Command Center located in New York that the umpires will call into. Other umpires will staff the Replay Command Center. This process has worked very well for checking goals in the NHL.
Seems like a good resolution to me. Everyone will be worried about delays, but for the most part we'll be looking at two replays per game, four at the most (plus any reviews of home runs, which don't happen that often). If it takes 90 seconds to review a play, that's three-to-five minutes of added time per game. That's worth it to me. It is possible that we'll see players delaying the action after a close play in order for videos to be checked, and I predict at some point in the future there will be a time limit on how long you have to challenge a play.
There will be pressure on the Command Center for a quick resolution. There will also be pressure on managers on when to use their challenge. Do you use it in the first inning of a 0-0 game or hold back for a more crucial moment? If the manager absolutely knows the umps got it wrong, I think you have to use it early in the game; if you're right, you'll get another challenge anyway.
Both teams will have access to a standardized video feed in the clubhouse. This makes sense; you have to ensure both teams have equal access to the same replays. Man, suddenly that job is pretty important. It will be interesting to see who teams have monitoring the video. A coach? A player not in the game? The video guy? That person will have to relay advice to the manager ... and he better be right. Doesn't sound like a fun job to me. Can you imagine giving the wrong advice to Tony La Russa back in the day?