MOP Skills / Talents:
1. Burst of Speed - This skill promotes a style of play with using cooldowns, and running to reset it which is quite frustrating for players to deal with. Even after playing Rogue myself I find shadowstep to feel much more rewarding when using it properly, and I feel this talent greatly contributes to a bad playstyle for rogues.
2. Angelic Feather - Makes priests run at mount speed, making it unnecessarily too easy to get fears especially with Spectral Guise. The only reason this doesn't seem like a big problem right now imo is the huge number of Shamans you see in arena, but Tremor nerf can change this in WoD.
3. "Trinket" like abilities and Talents: Unbound Will, Desecrated Ground etc. - I feel like these abilities are just far too strong in general to be balanced around.
4. Unpredictable Burst abilities: Lava Burst procs, Starsurge procs - These abilities do far too much damage to be unpredictable.
5. Instant Burst that results in nearly ~30% HP without or with little setup from a single character: Enhance Ascendance, Starsurge, chain Lava Bursts, Chimera Shot Crits and more - They can create ridiculous sudden deaths.
6. Damage Reduction Stacking Talents and Glyphs - Certain classes can stack a near passive 30% damage reduction, while I understand some classes need damage reduction more than others nearing 30% passively seems a bit ridiculous.
7. Stealth abilities such as: Subterfuge, Spectral Guise, perma Hunter invisibility, AOE Invisible from Rogue, etc. - These abilities create largely uncounterable events, and while having some of these abilities in arena is nice I believe there are too many in MoP at the moment.
8. CCs and Haste - This isn't really a talent or skill, but how Haste interacts with CCs. CCs get progressively harder and harder to stop as an expansion drags on. I feel like haste should maybe interact with CCs differently so that a Fear / Cyclone / Polymorph isn't almost 30-40% faster at the end of an expansion compared to the start.
Here's a few from WOD, this is based on a few hundred 2s games and dozens of 3s. Some of this may still be inaccurate, and may have already been changed without my knowledge as WoD gets patched pretty quick.
WOD Skills / Talents:
1. Sudden Death - This proc is making Warriors do insane amounts of sudden burst.
2. 8 Second Horrify from Shadow Priests - Don't really need explaining, but an 8 second stun that isn't on stun DR is insane. I've been told this may be a bug.
3. Double (70%?) Shield Wall from Druids - I believe this class already has too much defenses that has been carried over from MoP, particularly for Boomkins / Resto Druids that never had this skill to begin with. This makes Druids ridiculously hard to kill between their high mobility, and amount of skills usable even while they're in certain CCs (barkskin, displacer beast, etc).
4. Shaman Level 100 Talents - Saying this as a player that plays Shaman, these talents just seem horrible and extremely underwhelming in PvP no matter what spec you are. Elemental fusion could maybe be okay for Elementals.