Another big problem is the way - as Ssalarn suggested an alternative to - that the classes are all advertised and described as though they were equal.
"Be this Rogue" it basically tells new players. "You'll be the super sneaky guy that can do all sorts of useful things!"
And then a book over is this guy, the "ninja" that's better at rogue in every way. And both of them later discover that they're not the stealthiest thing around, the party spellcaster is. And they're not the best at traps and stuff, there's other people that are at least as good if not better before magic, and THEN they go and cast one of those spells they have like 30 of to work with that day.
"Be this Monk! You will be strong and cool and able to do all sorts of stuff and no one's better than you at unarmed combat!" ... Except, you know, everyone else that does unarmed combat better than they do, like certain fighter and barbarian archetypes, or a well built druid...
If everything was roughly as capable in terms of doing its own job as well as affecting the flow of the story, the minor details would be mostly inconsequential and left to matters such as just the roleplaying or build/skills of the player.
But as long as you've got classes that can barely - if at all - do what they're advertised as being masters of even with high optimization, saddled up alongside classes that can bend over and ravage any situation (and the plot) with little to no effort or understanding, like the campaign were some helpless village-girl caught by a horde of viking dragons who're done with the "burning" part of work, "balance" is going to remain an important bloody issue to deal with.