Tank, Healer, DPS. The three required classes of any class-based MMO. Which pretty much makes dungeon adventures very standard and boring after the first couple of times through. Wouldn't it be nice if you could take any class-combo you wanted into a dungeon. If there were ways to engage encounters that didn't rely on the tried and true, "You take damage, You heal, and You kill stuff" routine?
I think a lot of this is simply due to the lack of decent AI. The mobs can't think on their feet, so the standard is to simply make them do lots of damage, and give them lots of hit points so that the players have no options except to follow the rails on your amusement park ride.
But how could this be handled without providing numerous opportunities for players to find ways to cheat the system? With instanced dungeons, you could have some variables that changed the composition, layout and abilities of mobs and bosses based on the group makeup, but then you would have to lock the instance to that group, and not allow them to switch or add other players, which in turn presents a whole slew of problems on its own (not to mention the extra coding required).
In reality, I dont think anything can be done along these lines in a class-driven MMO without some major changes to how things are done. Hit points and damage would need to stop scaling exponentially with level. Players would need to be given lots of various utility abilities that would provide them opportunities to do more with less so to speak. Computer AI would need some serious buffs and the Monty Haul equipment hunt would need to be seriously scaled down so that it doesn't have such an effect on a character's power. In short, too many changes for a class-based system to incorporate. After all, one of the primary reasons for creating class-based systems is to compartmentalize players into easily quantifiable boxes that can then be stacked together and compared to the mobs for ease of power balancing.
When you design a game based around character classes, you're putting the train on the tracks, and your combat system will be pretty rote from there on. If you can come up with ways to make things less generic and more interesting while still maintaining the concept of character classes, then more power to you. But, in the meantime, I'm looking at the skill-based systems as the future of online worlds.