Any free MMORPG that doesn't suck balls and has enough players for a balanced economy should be pretty known by now, no? I suggest looking into more obscure MMORPGs like MUDs or something, there might be a few good ones out there.
Another suggestion is Ryzom. I like it because it has a very fantasy/sci-fi theme to it, is free to play and has great graphics, especially flora and fauna look stunning. Seasons last a few days each and have a visible effect on the world, and there are flowers and little critters everywhere.
I never got into the levelling, but it's a class-less system so you can specialize without being limited. Skills are fully customizable, as you can switch the effects and costs around. You can create a new skill that causes bleeding and extra damage at the cost of 10 stamina, for example. you have to balance effects and costs, though making them balanced against you gives you more xp for the relevant skill tree.
Resource gathering is very complex and detailed, perhaps more detailed than combat. Resources vary in level and quality and you have to scan them out with various skills to locate them, and other skills to actually pull them out of the planet.
Crafting is also quite detailed, the resulting item's statistics and colours fully dependant on the materials you used to create it.
Quests are sucky, though. Most of them are delivery, slaughter or gathering quests.
On a side note: I don't believe in the balancing of classes. Eventually everyone ends up with similar skills, or awesome-sounding skills that got severely nerfed.
Classes should be able to pwn at least one other class. Preferably in a circle.
Warrior pwns Mage pwns Ranger pwns Warrior. Warriors have the brute strength to withstand magic and just knock the mage silly, Mages can attack rangers from a distance and detect their sneakiness, and Ranger can sneak up and poison warriors with ease.
It makes for much more interesting grouping in PVP, and less likely for one class to become too popular, as there's always another class to pwn them.