Some of this might repeat what others have said, but for simplicity's sake I'll try to lay it out as a coherent whole. (Sorry for being a little long...)
For starters, the mage/cleric distinction is a funny one because it involves not so much the role
of the character, but where the magic comes from. It is tradition
for clerics to be thought of as primarily healers, but that need not be the case. I think the first question is to figure out where magic comes from in your world. The mage/cleric distinction assumes that you have divine magic (clerics) vs. some kind of natural "Force"-like magic (mages). Then you get psionics, which are another type of magic -- exploring the "physics" of psionics vs. "normal" magic would be an interesting exercise, if you think about it.
The point of the above paragraph is that I'm not sure it makes much sense to differentiate mages and clerics based on the source of their magic. Instead, it makes sense to differentiate between offensive and defensive 'magic'.
That said, let me try to answer your question.
Mages (offensive magic) -- these characters are weak physically but can seriously mess with the opponent. That is not limited to dealing damage; in fact, arguably the most powerful spells aren't the damage dealing kind. Spells that remove protections, weaken attributes, confuse or charm are the kind of things I am speaking of. And of course, there are the standard blasts, booms and zaps that seem to be a staple of the genre.
Clerics (defensive magic) -- basically the opposite of the mage, in many ways. Instead of trying to hinder the enemy, you help your friends. Adding protections and resistances, increasing attributes, and so forth are examples of this. Healing is a staple (but I don't see why healing should be limited to the divine).
Both kinds of magic user typically require escort to be worth anything in battle. I believe that a D&D cleric's ability to take moderate amounts of damage is offset by their relatively weak damage-dealing capacity, and relatively smaller set of available spell effects.
Warriors -- these guys are more interesting than they get credit for, IMO. On the one hand you have the very defensive kind whose main job is to take punishment and keep on smiling, while not necessarily being primary damage dealers; on the other hand you have berserkers who run in screaming and inflict massive damage but are also fairly weak. The thing is that the second kind is sort of redundant with offensive magic, other than the point that one uses fireballs and the other uses weapons. But that is an interesting redundancy, as it allows you to have similar roles yet rather different experiences.
Anyhow, I picture warriors as being the most self-sufficient kind of character, able to handle fairly well every single situation except for just one: offensive magic. A party of several warriors could be defeated by a single mage (possibly with an escort) who judiciously uses confusion/charm type spells to prevent the warriors from doing what they do.
Although one often hears archers termed warriors, I tend to think that they are not the best examples of warriors due to the fact that they avoid combat -- they fall more into the category of rogues, which brings us to...
Rogues -- probably one of the most complex characters, at least in my opinion. The common conception is that a rogue is a thief with a strong accent on actually stealing stuff. I think that is a something limiting point of view, because it leaves out a host of other possibilities and nuances. The term "rogue" also implies a certain amount of lawlessness, which I think again tends to bias people in directions that aren't necessarily the most productive. But for lack of a better word at the moment, I will stick with "rogue". (Morrowind calls the category "stealth", but that is a category name, not a class name...)
Let me try describing it this way instead. Whereas the warrior is interested in direct confrontation with opponents, the rogue is interested in indirect confrontation, or perhaps even entirely avoiding confrontation. That could mean attacking from a distance, avoiding detection, setting (and disarming) traps, poisoning, subterfuge and disguise, bluffing, etc. One way, perhaps, to say it is that rogues are more subtle than their warrior counterparts, and in fact must be subtle in order to survive.
The problem I have with this is that it is very hard to translate into a MUD-type game. What exactly does it mean to "trick" NPCs or bluff your way past them? Single-player games allow you to bluff and trick by presenting dialogue choices where you might or might not convince the character you are talking to. But persistent-world multiplayer games typically don't have much interaction with friendly NPCs, let alone potentially hostile ones.
In single-player games, a rogue sometimes gets around by avoiding confrontation completely, e.g. by sneaking past opponents. Since the objective in those games can be more mission-based, it makes sense to avoid enemies if you can still accomplish your goals. But how does that translate into a persistent-world multiplayer game where the goals are not really storyline- or quest-based? Do you sneak past the dragon to pick up the treasure it's guarding? Can you do that over and over and over again? Is that even fun?
I'm somewhat philosophically opposed to in-game theft because its social consequences are too negative. In the real world, there are important consequences for getting caught when you steal, or back in the days, getting killed when caught (by the victim or the legal system). Therefore there was at least some incentive to not steal. On a MUD, that incentive is much less powerful. You die; well, ok, too bad, so sad, respawn and you're done with it. You can even avoid having your equipment looted by not wandering around with it. For other consequences to be meaningful, you need either a very, very active human monitoring staff that can handle these incidents (which is not an efficient use of resources given the relatively small payoff), or an in-game law system. In-game law systems aren't perfect, but actually aren't so bad, all things considered. Anyhow, you still have the problem of being able to all-too-easily remove very hard earned items from characters simply because you happen to have chosen the right class for it. Basically, the role becomes one of a poacher rather than one who accomplishes things.
Let me try to cut this short because it's already longer than I meant it to be. The short version is that the "rogue" is a character that relies on everything but direct confrontation as much as possible; this typically means relying more on wits and guile than brute force. The rogue tries to, and in fact needs
to, be clever, whereas the warrior can get away with a more straightforward approach. But what, then, is the real purpose of the rogue in a multiplayer game? That is an interesting question.
The simple answer here is to make the rogue a support class that disables targets physically with things like poison, gouging, ranged weapons, stealth, etc. But I'm really not sure this is a good answer...
In the end of the day, I think that if you want to box people into classes you need to be very careful about the roles you choose for those classes. Sometimes decisions will have to be made that don't "make sense" but are necessary for game play. For instance, it doesn't really make sense that a rogue can hamstring an opponent but a warrior couldn't. (That's a common difficulty: differentiating between the combat roles of a warrior and a rogue.) For these reasons and others, personally I am leaning towards removing classes and letting characters develop "free-form" as in the Elder Scrolls games (at least Morrowind and Oblivion, the ones I have played).
While I know this didn't really answer your question, hopefully it provided some food for thought...