EDIT: Actually, I wish there was an MMO where the classes differ in ways other than combat ability.
For example, in WoW, every character has equal crafting potential. There's probably minmax number-crunch things you can do for optimal efficiency that I don't know about, but basically any character can craft anything, given that they keep up with the type of materials they get.
In contrast, Runescape makes it so that getting a high level in anything (besides combat skills) is pretty arduous work. Nobody's going to sit around grinding Cooking all the way to level 100, so the majority of players would just pay another player who has a high Cooking skill to make what they want. It created a neat in-game player-based economy. These are gameplay experiences circa 2007 though, so RS has probably changed quite a lot since then.
Speaking of crafting, just recently (in my ongoing internal construction of my *ideal* MMORPG, one that wouldn't bore to me tears) I thought of how I'd implement crafting if I were to make an MMO.
My biggest beef with crafting, is that it's a boring task implemented into the game to occupy the time of boring people. Where it's always simplified down to A + B = C, often with a level requirement barring you from even doing that. That doesn't sit well with me.
So I got to thinking, how would *I* do it? Well, first I'd remove the level cap, then second, I'd make any and all crafting systems pointlessly, arbitrarily, and maliciously complex and difficult. So difficult, in fact, that anyone used to ordinary crafting systems would find it completely impenetrable. So complex, that each form of crafting would require it's own dedicated fan-wiki. I'd want anyone who is half-hearted to be instantly disgusted with it, so they'd go and occupy space at the more viscerally exciting portions of the game, and I'd want anyone who is truly whole-hearted and curious to be enchanted by the effort and thought that is needed to explore this road of experimentation and be wholly fulfilled by the intellectual labor of it.
To give an example: suppose you wanted to make a health potion, undoubtedly the most easily recognized and ubiquitous of all potions. It would require gathering the ingredients, mashing, grinding, boiling, measuring, adding reagents, filtering, then distilling. Just a set of delicate procedures that must be carried out in a specific order. It's something that would require studiousness, carefulness, and math. Someone who is very skilled at it could learn the inner workings of the actual chemistry that the game is simulating and refine it, being able to produce either more with less material, or produce a better product with the same materials.
THEN, ideally, the end product wouldn't be named "Healing Potion", it'd just resemble a red liquid in a bottle, completely identical to any other red liquid in a bottle. A careful alchemist could use a labeling tool to personally label it 'healing potion', but otherwise any player can't take it for granted that a red potion equals a healing potion, as it could be ANYTHING in that bottle: a fire potion, acid, cherry juice, or just water with red dye in it. I'd think that if any player wants to dodge the high prices of a guaranteed NPC potion merchant and acquire their potions from cheaper 3rd party PC potion sellers, then the chance of getting conned with fake goods, cheated with watered down goods, or being disappointed by just plain poor quality goods should be an omnipresent risk. Whereas trustworthy alchemists that have taken the time to master their trade and consistently deliver both high quality and high diversity of potions could make a name for themselves, make the trade lucrative for themselves, and find it personally rewarding from the hard work and thought required.
I feel it'd just be plain more respectful of a player's intelligence and time spent in the game rather than making them click on rocks and haul them for exp gains.