Again, you aren't getting it.
In the first place, people will play what is fun or they will stop playing because they aren't Having fun. It's that simple.
This is my point, people will not enjoy a game they cannot feed their internet Ego, by being the "best" (normally at dps) or whatever job they do. So if they can do nifty things outside combat like scalling walls, yet cant do shit for damage, many people will be turned off playing those classes, forcing many more of Class X in the system imbalancing the game. A huge flaw in any game. This is why classes need balance, if not people always float over to the "best"
But what you really aren't getting is that having quests and things to do of the nature that I describe doesn't preclude the stuff that already exists. So inclusion doesn't break anything. It only adds content, and much needed content at that.
Again, you are not getting that so much content on release is already there. For them to add extra stuff that will be class based, some content must be removed. Otherwise the game gets too big on release and would take too much longer on working the content in, because of the nature of game-making time.
Which means all this "extra content" takes away from the overall content in size, so even though the game feels more unique, it has actually shrunk in return. Remedy of this is to fast release expansions. But that isn't wise for an untested game.
Why not just play a single player game if content will be pushed just for single people/classes?
And you are falling back into the "Every player needs to be able to do everything and go everywhere in equal measure." which breeds "Balance = same". Only it doesn't and doesn't need to.
That is only because the playbase will always drift over to what is best. They always have. The class representation numbers on WoW each 3 months shows it quite often. Most of early cataclysm Shaman healers sucked because of horrible balance, they practically dissappeared.
Plus if I was to kill this general in some fort, would I take the rogue who climbed a wall, opened a door for us and does blah for damage. Or take another Fighter/Warrior/Facesmasher, and take the "hard way in" killing enemies reaching the general and healing befofre combat to make the General fight easier with the better damage dealers? MMORPGS have all become about the battle. Not the puzzles. This isn't paper DnD or single player games in which puzzles were plenty and hard where class difference could shine. Even MMO DnD doesn't do puzzles. That just doesn't go over well with most players in the genre. (casuals)
And then there is also the DPS that you mention. This is seen as the holy grail of games of this nature. And 100% precludes games like Thief, which was massively fun and at least moderately successful. Tying to DPS for every character means tying to Combat ONLY. Give the players a choice. Give them a chance to excel and achieve in the same measure and degree without combat and they will. Give them other things to do besides JUST straight on Combat and DPS becomes less important. Stealth action is a genre into itself and so is a popular alternative. So I am sure would spell craft. And it doesn't need to be just the mainstays of RPGS today (Wizard, rogue, Fighter, Cleric). There are as many ideas and possibilities as there are character ideas. Stop trying to pigeon hole things to fit a mold.
but in the end, it does come down to the people who make these types of games. they are programmed to believe that everything needs to balance out to exactly the same. Every class at every level needs to have a common DPS. And so Innovation dies. Add to that the fact that they are playing to the lowest common denominator and innovation is truly dead. That is why TOR plays so much like WoW. That is why City of Heroes plays very similar. And all of the rest. Not because they HAVE to be that way, but because people equate balance with Same. They aren't.
Again, as I said MMORPGs are successful "today" because they tie to combat only, and most people find that fun. The only way to make classes unique
and enjoyable to the majority playerbase (casuals) is to give them unique roles in combat, which however majority of MMOs already are... Warriors don't shoot fire, Mages don't use 2-h swords, Rangers use bows and such but don't do magic. However doing this A) Disrupts balance, making it hard to balance. Look at WOTLK DK + Hpal 2v2 arean team. Basically unstoppable. B) Giving classes unique jobs in a fight (sneaking, picking, magic barriers, knowledge prior to combat) would cause great issue inside the Holy trinity universe, making group gathering too hard, when just tank, healer, dps is hard enough and relies on too many people.
I am not saying your idea wont be fun (because it definitely could be), I am not saying your idea wouldn't work (especially in a D3/single-player game), I am saying that the idea of class uniqueness does not fit in the MMORPG genre today. Maybe if we went back 10-12 years just after the release of EQ and DoAC, and Asheron's Call we may have been able to get people interested in this. But Because of WoW, the genre has morphed into a pure combat related game, and the majority of the community will find no fun in trying to drastically change that. Especially since most of them are casuals and/or children that just want to play, kill things and feel good about it. (Psycologically it makes a person feel better feeling "strong" in a virtual world. Game psycology books have shown lots of evidence of this recently) Not try and play using class restrictions and puzzles being a strong point of a game.