Specifically the fact that the enemy dragons get to use their ranks now, so the difficulty of the battles has increased MASSIVELY. The Leagues are much harder than the Arena now (since you don't get to see the upcoming dragons or skip) and there's no longer any good place to rank up new dragons. Plus it's much easier for one bad matchup to completely block you from progressing in the Leagues at all now.
I predict League chests and League requirements in races will now be off-limits to anyone who doesn't already have a good selection of L40 A+ Heroics.
Horribly broken and completely unjustifiable IMO.