Semi-random, long winded thoughts. I was going to post this the Warcraft forum, but figured players of other games might chime in if it were here.
There are things that MMO's naturally excel at, and other things that they're not so good at by design.
1. Players themselves are the content
You read that correctly. Your ability to interact with other people is the strongest asset this genre can possibly offer, and is exactly why many titles that are multiplayer have shelf lives that extend far past that of what a single player game offers. Those players are around long after bad guy #46 is dead, long after any small story arc is finished in the game. Creating scenarios where those players can interact with, or against each other, and ultimately make their own reasons for logging on is what good mmo's are made of.
Insulating players behavior, limiting how players can interact with each other, and creating an environment where the game can be largely consumed in solo fashion is a losing effort. It puts developers in a scenario where they have to create content that has a generally low skill cap, as you will have a wide range of players participating. It also creates problems in trying to judge overall difficulty in a given area. 15 enemies in a small space of land might be cakewalk for 15 players, but a few months later, that solo player walking through that same area might get overrun. While some games try to partially address this with mob scaling, in my experience it doesn't work out all that well.
2. Your avatar is your representative
There's a value to persona in these games, one that is weakened when players can rename themselves, move from one server to the next, faction change, etc, or even have a multitude of alts. You're no longer 'you', you're simply that one elf over there. You can't play the hero and have much in the way of recognition for it. You don't get to play the role of an antagonist and gain a level of notoriety for it. You can't act even worse, and then appeal for redemption from your peers, or vice versa. It weakens your position in the world you're playing in as an individual player, and again to point 1, lessens the strength of other players as a primary content factor.
3. You are not a hero
You being *the hero* is a doomed initiative in an MMO... because everyone else playing is being fed that same exact line. And unlike a pure single player environment, you're seeing the results of that on your screen, which sort of invalidates everything. Lets see.. I'm level 4 with a leather belt and rusty axe, and this NPC just called me his champion. I wonder what he calls that level 40 that just rode past with the full armor set?
4. Leveling as a means of access rather than power
This is a big one. Leveling is a staple of the genre, both for MMO as well as single player RPG's. The problem is that leveling by its nature effectively shrinks the world that is going to provide you with entertaining gameplay as you go. I'd like to see a game where leveling was almost solely a means of access to areas rather than one of access + an often significant jump in player power. It keeps your effective game world far larger, arguably making the game bigger as you go. Typical leveling also wreaks absolute havoc with PvP.
5. Large amounts of freedom trivializes overall gameplay
I've talked about this a lot, so don't want to repeat much here, but even the smallest of limitations in a game force a player to make a decision about how to handle it. Removing those small decisions increases a players sense of freedom, but also reduces the sense of involvement with the character. Limiting or eliminating risk has the same effect. Neither is good for this sort of game.
There's more to life than combat.
I'm not sure I've seen any MMO create a role where a player isn't a fighter first and foremost. Generally such things are left to player interpretation, which is fine, but there's room for that sort of thing to be part of intended gameplay itself. An example might be dowsing, where you've trained to find water, but aren't adept at combat... so other players need to protect you while you find the water (which in turn benefits everyone else in your group, etc).
Flame away, add your own, etc.