Cataclysm, even more so than any previous expansion, really redesigned how classes level up and their basic functionality. Some classes, like paladins, saw an entirely new resource mechanic. Others found themselves turned away from previous core concepts (like a death knight's ability to tank or DPS in any tree) or given a more clearly defined role from the start.
In the process, while many players had to relearn their classes, someone like me can come along and try again on a class that feels much more fluid and dynamic to level. I have started 16 druids over the years, only to delete them by level 20, so the sleek, compact redesign of the class was a revelation to me.
This, of cours,e does beg the question of the inverse. If a redesign makes the class easier to pick up but turns off the long-term players, did we gain or lose something?