A well designed game does not require the player to choose not to use certain features in order to present a challenge. Think about time honored games like Chess, Poker, Billiards, or even more modern ones like Risk, Monopoly, Settlers of Catan. These games are great because of the way they are balanced.
No chess player would ever tell you that the way to make the game challenging is to avoid using the knights, or queen, or one bishop. You don't balance the challenge of a game by choosing to avoid using certain game mechanics. You balance the challenge of the game by finding an opponent to suit your skill level, and then you bring all the tools of the game to bear against that opponent. In Skyrim, the "opponent" is the difficulty level. There is no difficulty level that is challenging for any player that uses all the game's features. There should be.
Well, I actually used chess as an example at another point. People often do handicap themselves against less-experienced players by giving up a queen or other piece, but the point is - who actually wants to do that? If you have two potential opponents to play with, assuming no outside influences like one is a friend or something, and one is on you level and requires no handicap to present a challenge, and the other is a good bit behind you and requires you to give up a queen or rook or bishop in order to be challenged, who would choose the lesser player? Nobody.
And that's the problem, apparently, with Skyrim's difficulty: people are having to handicap themselves even on Master difficulty just to get a challenge. That isn't me, by the way. I'm playing on Adept with 100 smithing but no enchanting, and it still presents a challenge. Elder dragons can take half my health with a single blast, and that's with me wearing legendary steel plate and a heavy armor skill in the eighties. But others clearly have found that they have to handicap themselves to be challenged even on the harder skill levels, and I find that unacceptable.
Now mind, this is by no means an overwhelming criticism of Skyrim in my book. I'm a rather outspoken fan of Skyrim, and I suppose I've probably ticked off at least a couple Morrowind fans in debates at times, but I have to admit that it sounds like game balance is a legitimate issue with Skyrim, at least in terms of overall difficulty. I still think the people that say Destruction is underpowered are just doing something wrong, because I've also seen a number of threads about people saying they're conquering the world with a Destruction mage.
So if a fairly hard-core fan of Skyrim like me is supporting those who say there's a balance issue, I figure I'm being reasonably objective about it, since it runs counter to my usual (knee-jerk) tendency to dismiss a fair portion of criticism of the game as either whining or else nostalgia for Morrowind.