And another thing that makes no sense at all. Why are people going on about it takes a mage X seconds to kill a mob but a melee takes Y seconds therefore it's bad?
What on earth makes you think only X = Y is balance? How on earth does that make sense? By that logic the fact that a sneak kills in one hit means both mages and melee need to be adjusted to kill just as fast!
Different classes play different. Different builds kill things at different speeds. This is a good thing and has absolutely nothing to do with balance. It means that choices we make actually results in differences in how you play and how combat plays out. Otherwise the game would be so incredibly monotonous and boring.
Sneak kills are balanced because they can only be done once (generally). I believe that the perk that lets you keep re-sneaking and re-doing sneak attacks is broken because it removes the element that makes sneak attacks balanced. In the same way, magic is usually balanced by a high risk, high reward philosophy. Magical spells deal a lot of damage, but in return mages are squishy and prone to dying. Skyrim got the squishy part right, but it didn't get the damage part right. Melee generally do less damage than mages, but that is balanced out by the fact that they are hard to kill. In Skyrim, they are both harder to kill and do more damage. See how that breaks the system?
So yes, different classes play differently. And different should mean exactly that. In Skyrim, though, different means weaker, at least with regards to destruction and mage damage.