I speak only with experience in the level 70-74 bracket but I have much experience in it. I'll start with the classes that need to be tuned down. First off mages, if played correctly they are the most dominant class in this bracket. They have multiple stuns, multiple disorients, a few silences and pack a mean punch with a strong shield. I dueled 2 different ones on my level 70 paladin and won after 15 minutes each. They can be fixed just by taking away some of the cc or if blizzard doesnt want to do that, take away about 75% of ice barrier's strength. They are that strong in every aspect.
Next up is paladins. They nerfed the damage which is good, but next is to just tone down the healing a little bit so that they may die at some point haha. I play semi-conservatively and havent died in roughly 15 matches with 10+ killing blows each match. The damage is good because it is subject to being locked out and must be used at the right times.
Next up, priests (Finally). Very strong shield coupled with very strong heals and very strong dps. Very strong may even be an understatement. Basically just need their damage and shield toned down, shouldn't be a hard fix. Healing in general needs to be toned down for the most part, many classes are becoming close to invincible, especially the next one.
Druids.... The resto druid is an abomination, coupled with fairly weak damage the resto druid can pump out insane healing with their hots to the point where one would need about 4.5k dps at level 70 to take one down. Just nerf the hots by about 65% (that much) and give them a bit more firepower then they're good to go.
Hunters are only going to be included because of their hella nuke in aimed shot. A little damage nerf and problem solved.
Unfortunately I have little to no experience in which classes are underpowered. I've played my paladin since 5.0 and have always been in the top tier. I know warriors are actually fairly good in the 70-74 bracket and I tend to dominate shamans, death knights, and warlocks the most. Well I hope this helps out a little bit. Honestly I think everything could be fixed with toned down healing, toned down shields (ice barrier, pw:s, etc.), and a few less cc's (ESPECIALLY mages). The damage would be at a good place comparatively if the healing would just be toned down