As a Shaman, by far my biggest GCD complaint is Ancestral Guidance. You ramp up your damage in order to heal up a dungeon group and that 1 global feels like an eternity. It’s not a huge performance issue but it personally really turns me off from the talent due to the bad feeling alone. Of course there are a few other shaman abilities that don’t feel great but AG is by far the worst.
As for other classes, here is a list of particularly frustrating gcds off the top of my head.
Innervate: As a boomkin it’s disrupting to the rotation without feeling like you accomplished anything with the time spent. As a restoration druid it’s maybe a bit better since you directly benefit from the global, but as a general rule I dislike GCDs that don’t do anything on their own. I’d feel a lot better about innervate if it also increased duration of hots, healed yourself for a moderate amount, etc.
Incarnation, Tree of Life: In PvP in particular this GCD feels BRUTAL. Not only do you need to press the GCD to enter the form, you also want to innervate as the next GCD since regrowth spam is mana intensive. Even without the Innervate it’s rough. We’ve had many very frustrating arena losses due to my druid coming out of a kidney, hitting tree and either dying during that GCD or dying after getting 1 regrowth out because it just wasn’t enough. This is another example of a GCD that I feel should either not exist, or do something immediately. Perhaps the CD could immediately apply all hots to current target, or even just a swiftmend, anything to make it less “empty”
I may return later with more but it’s late so I should stop here for now. As a quick summary, I strongly believe the game would feel a lot smoother if every ability that is on the GCD had an immediate affect. Think of things like Darkglare increasing dot durations, metamorphosis giving you a leap/stun. If Elemental Shaman Ascendance refreshed Flame Shock on use, I’d feel totally okay with it being on GCD, but as it stands it feels like a road block of sorts. “Empty” GCDs don’t feel very rewarding to use, and for healers in PvP can have frustrating consequences that don’t result in an engaging experience.