16 was the first version with the MLB licensing, 17 will also include the MLBPA licensing which means the true high resolution pictures facegen style of all the players, they will show emotion and aging. Another big one is the Historical minor leagues now included in 17, which means if you want to start a league lets say in the 80's, it will include all the minor leagues with the real players and they will also reproduced them every year after that. If you like playing the MLB (not fantasy which means a new baseball universe that you create to your liking) it adds a lot of realism. You also have the choice to play GM and Manager of Manager only (which means you need to put up with the GM AI and the trades/signing/promotion that he assigns to your team).
Another big one for 17: "Smarter, more cunning AI GMs and managers, with individualized personal approaches to roster-building, scouting, and finances".