First and foremost in game management.
Secondly it allows historical leagues easily with the import of the Lahman database.
Much more in depth game play. I played Mogul 2K, then the demo for 2K2, in my opinion that version of the game was the same or only slightly better as Mogul and while it was fun, it just wasn't in depth enough for me. I heard of OOTP, tried and was hooked. I like to have more control of my team as a GM; I like more details of managing the minor league levels, I like having to worry about the difference between minor league and major league deals (rather than default 30K (1pt I think) deals for rookies). I liked being able to easily set up fictional leagues, I liked being able to edit the statistical era of the game, in general I just liked OOTP3 more. From the testing I've done for OOTP4 its a huge step forward.
I don't know how well I've articulated this. But for me this game was much better than Mogul. Mogul just didn't have as many options that I was looking for. (I used to spend days trying to set up a fictional league, just the way I wanted) To me this game was just better.
