Took away the utility from the Shadow Priest as far as their "mana battery" function was concerned - but now they can actually do DPS.
Street reminded con-goers that there were actually two other talent trees for the Priest! Wanted to make Discipline a viable raid healer in a way that's different than Holy, revolving around single-target heals and damage prevention like shields and Pain Suppression.
Thought Priests could use a new defining DPS ability, which resulted in Mind Sear - a spell that does damage to a target and everything around it in a minor AoE. While doing a run of the "new" Naxxramas raid instance, the tanks died, people asked "Who's healing?" The Priests responded, "Uh, we're all Mind Searing."
No more Priest Racials! Didn't want it to feel "Oh, you NEED to have a Dwarf Priest," or "Oh, don't be that race, they suck."
Frostfire Bolt - Rather than always only having the Fire, Frost, and Arcane guys, the intent behind Frostfire Bolt was to maybe help form the Elementalist build that people have been wanting. When they first announced the Mage changes for WotLK, they were lacking a cool ability at the bottom of the tree... hence Mirror Image.
Now, they have 3 raid-viable DPS builds ... maybe 4 if they can get Elementalist builds to where they would like them to be.
"Druids, we feel could use a lot of work." Lots of boo-ing.
"They're a bit overrepresented in arenas, for example."
Moonkin had intentionally lower DPS, but brought lots of things to raiding ... like running out of mana so they couldn't do anything. Their DPS has been buffed to where it's competitive; the improvements made to Entangling Roots allow them to properly CC in instances.
Resto druids... (more boo-ing) Everyone knows they have a niche for HoT (Heal-Over-Time) healing, but that was all they could do. So the developers gave them other abilities like Nourish, which is like Flash Heal, used to top off a Rogue near death (if you want to keep them alive). While not as "core" as Chain Heal or Circle of Healing, Wild Growth is the AoE heal for moments where the whole group is taking lots of damage.
Also, they finally gave Druids an out of combat resurrection.
Blizzard took a look at the Feral tree. The Ferals have always been the hybrid tree - you can tank but not as well as a Warrior, you can DPS but not as well as a Rogue. So now they specialize it a bit. If you really want to tank, you can grab the Bear abilities, but you forego some of the cat-form ones, and vice versa.
Totem Consolidation. Totems are a cool and unique mechanic, but it's never quite been where we wanted it to be. Did a lot of work to make totems more functional, to help them survive a little better for instance. Blizzard wanted to fill some holes in the Shaman repetoire with stuff like Lava Burst for Elemental, Riptide for Restoration. They also finally have a legitimate CC ability with Hex.