Tbh the most important factor of Fifa 15 is if they get the pretty powerful element of randomness and chance of 14 out of the game.
Of course luck and random outcomes of certain situations are a part of the game, but in 14, I largely had the feeling that most of the times, no matter if I lost or won, I couldn't really attribute it to either my errors or me playing well. Against a clearly inferior or superior opponent this was of course not the case, but in any game in, to say it in Fifa terms, division 1 & 2, where I spent most of my time playing, every goal was at least to a rather large extent, much larger than in any previous games, the result of luck, and not really the abilities of the respective players.
What's also really annoying is Fifa balancing out strong differences in ratings between two teams facing off against each other, as it makes UT basically unplayable after your team reaches a certain overall rating.