It's the healing. Before you could throw a single die multi times, eating opponents dispel die etc..
But now, you cannot risk throwing a single die becuase if you fail to make the cast value of the spell, that wizard is done for the turn.
Also, none of the spells are able to be duplicated across wizards so the threat of multiple winds of undeath, multiple curses, mutiliple van hells in no longer there...
They went from a tier 1 over powered army to middle/lower end of the pack.
Still a great fun army to play, dont get me wrong, just their magic has become unimpressive.
I believe George Takai put it best when he said "Oh my".