In any statistical observation, removing the variables/noise from a data set is of primary concern. This rings true in baseball analysis as statistics try to isolate meaningful (and sometimes meaningless) parts of a player's game.
On-base percentage, slugging percentage and weighted-on-base-average all tell us about the results of a grouping of a player's at-bats, but they don't tell us anything about the process. BABIP gives us a way to put these statistics in perspective by trying to account for luck, but it's still focused on the results.
Enter the newest reduction: Exit speed.
Not to be confused with bat-speed (the speed at which the bat travels through the zone) exit speed tracks the speed at which the ball is traveling as it leaves the bat. This tells us something about a player's raw power (how hard they swing) and about their eye (better contact makes the exit speed higher).
Most importantly, exit speed is telling us something about the process that leads to singles, doubles and dingers. In short, a player that routinely crushes baseballs has value.
Hanley Ramirez is a player that hits the ever-loving crap out of baseballs and he has turned that in to a very successful major league career. We don't have the data on exit speed for all balls put in play, but ESPN does keep track of exit speed numbers on home runs.
Ramirez has hit 18 home runs this year and 15 of them came with an exit speed of over 100 miles per hour. The rule of thumb is that a ball hit at a certain angle (30 degrees, let's say) with a speed of over 100 mph is a home run most of the time. Other factors like wind, the ballpark and altitude play a factor in this as well; but the point is that if you have hit the ball over 100 mph, you've hit it pretty darn good.
This isn't exactly a newfangled way to think about baseball. Broadcasters refer to how hard guys hit the ball all the time - and as baseball fans we know that sometimes balls are smoked right at the shortstop for an out while other times a lazy pop up falls in for a hit. BABIP tries to tease out some of this information out, but looking at the exit speed can cut out the middleman and cut straight to who hits the ball harder more often.
Where this data is most interesting is at the minor league level. Not all teams are tracking this sort of thing (yet), but in leagues where the statistics mean next to nothing (the Pacific Coast League and California League come to mind) a statistical way to analyze tools is very valuable. Certainly scouts play a role in all of this as well - but pairing raw data with a scout's report doubles the confidence an organization can have when making a move.
As with all data, exit speed comes with a few caveats. Knowing that a player consistently makes hard contact is only half the story - on what pitch type is the player crushing the ball? Does he wait for fastballs, a common practice in the Cal League where pitchers have trouble with their breaking stuff, or can he drive curve balls as well? Is he only swinging at pitches on the inside part of the plate? Can he put a charge in to balls on the outside edge?
All of these questions can be answered with data provided by the same programs giving the exit speed; if/when this data becomes available for public consumption it will be important to pair average exit speed with pitch type. All of this data working together paints a more clear picture of the type of hitter a player is/can become.
On its own, exit speed isn't enough to evaluate a hitter; but as a companion to a thorough scouting report it gets us a lot closer to the right conclusion.