Diablo 3 is getting a bunch of balance and content changes in patch 2.2. I don't play enough to be able to comment on the overall shape of class balance but I do think Blizzard is in an interesting place when they try to make changes to the game at this point. The situation they are in is they have two very distinct games, both of which are called Diablo 3. The first game is one in which characters put on whatever gear they can find and that gear mostly just makes them a little tougher, do a little more damage, find a little more gold, or other similar bonuses.
Eventually those characters make it to maximum level and then everything totally changes. This second game is one where you can find two pieces of gear that have a set bonus. That set bonus gives you a permanent set of 8 minions who deal a truckload of damage and are extremely tough. If you are otherwise just playing a character who casts spells at the enemies you would find that these new minions multiply your damage by 3 times, making your personal contribution pretty pathetic. Also because you are now surrounded by dorks your survivability goes through the roof.
So we have one game where all the spells do something and people have a ton of ways to play, and one game where the only thing that matters is which set of ridiculous gear you are wearing. Any spell not supported by an overpowered set is worthless in that second game. If the devs increase the damage of Acid Cloud by 100% that would make it amazing for levelling up, but it would still be completely irrelevant in the endgame because it doesn't have a gearset dedicated to propping it up. This makes it tricky for altering gearsets or spells because there are two totally separate worlds that they have to live in. Much like it is always a challenge to build abilities in WOW that are useful but not overpowered in both PVE and PVP this makes new designs finicky.
This system is also very sensitive to small changes in the system. If a particular set completely defines a class in the endgame because they have no other comparable options any tinkering with that set is tantamount to tinkering with the whole class.
I should note here that Blizzard certainly could just throw up their hands and decide that balance isn't a big concern. Certainly the levelling game is ridiculous at this point as I think most people just get a max level character to powerlevel them from 1-70 in an hour so balance in that area is pretty irrelevant anyway. However, because Blizzard really wants Diablo 3 to be a competitive game and they want to maintain leaderboards and such they seem to have thrown their lot in with making things balanced between classes. Clearly they aren't trying to make spells or gear balanced at all, but they do want every given character to have a chance to be good. They know that players care, which I can attest to as when I was playing and Demon Hunters were the best class I generally found that 50% or more of the characters I ran in to were Demon Hunters.
WOW has become a lot more predictable and staid in terms of the way gear works and is handed out. No more outrageous items that last for five tiers of content, not any more. Diablo 3, on the other hand, has gone the opposite way and has swung from boring gear with pure numbers on it to complete insanity. I suspect it is better now for both games as the genres they occupy are quite different indeed.