I think the whole point of classes in RPG games is to impose a set of restrictions on the player, and to set them the challenge of doing well within those restrictions.
This isn't a bad thing. Really, it's what games are all about. It's why every single game in the history of the world has had rules - to restrict the players and see how well they do within those rules. In chess, you're only allowed to move each piece in a certain way. You're generally considered to have failed if you step outside the rules (for example, by moving the King at high velocity into your opponent's face
).
So, you create a mage character. Do you understand the strengths and weaknesses of that character (the restrictions imposed on you) well enough to win the game? Can you also do well with a rogue character? If not, can you do better by learning the rules better? Classes set a challenge for the player, which is part of the fun in those RPG games which have classes.
However, classes in RPG games rather assume you're investing enough time in one game (or one rule set) to really learn how to play several different characters. If you don't have the time or inclination to repeatedly replay a game, then classes can be burdensome. Rather than being a challenge to overcome for the fun of it, they become an obstacle to having fun. And if they're implemented badly in the game's rules, then they're just no fun at all.
Skyrim has ditched classes. The challenge in Skyrim is to play well within the restrictions the game imposes on
all characters. Players can, if they wish, impose restrictions on themselves - and can, if they wish, cheat by breaking their self imposed rules if those start getting boring.
Hopefully Skyrim's rules on levelling faster by investing practice in a few skills, along with the fact we can only select a subset of perks in any one run-through, will reward those players who like to replay the game - but won't punish those who are only going to play it once or twice.