Call of the Ancients: The strength of WotB is clear. Earthquake is appropriately powerfurll. CotA, though, really needs some love to be as good as those skills. First of all, my dudes should have full CC immunity. It sucks to bring out my posse, only to have them feared/frozen/jailed for half their duration, watching them do [Farley] JACK SQUAT! [/Farley]. They should have full pet survivability, if they do not already, it's kind of hard to tell how much damage they take because we do not have their health bar in the upper left like WDs. They should draw more aggro, or at least Talic (the dude with the shield) should. WotB has a defensive component, and quake works great with lifesteal. CotA should have a survivability factor as well.
Ancient Spear: Whoo boy, if D3Database is any kind of accurate, this may be the least used skill in the game. The reason is simple: it does not fill any need in the barb's toolkit. I'm using it right now, trying to see it's usefulness, and while it is hella fun to spear a succubus/morlock/fallen shaman, I would be better served by other skills in that slot. This one might just have to be replaced with something else, probably in an Xpac.
Monk: One with Everything: Obviously broken, but it can't be significantly changed without screwing over all the monks who have built their entire gear sets for this one passive. Most likely to be changed in the Xpac, when everyone will need to re-gear anyway.
Monk healing: Their healing skills need to scale with something. A good candidate for using the bonus to health globe affix to boost some skills.
Wizards: They are stuck in a catch 22 with many of their skills. They have to stand still to cast, but they can't stand still or they get smooshed, so they need to kill mobs to not get smooshed, but they can't stand still to cast, etc. This is more an issue with the class as a whole. I have no ideas for it, but it needs to be addressed.
WD, DH: What needs to be done to build skill diversity with these guys? I don't know, I've barely played them.