A sandbox MMO:
- Modular roles rather than classes, you pick/build/spec a role (tank, heal, etc) with various skills/passives that you collect/gain (skill tree? open skills)
Example: You have 500 skills with pre-reqs in terms of stats, other skills, total power. You select a number of skills ("Knightly Prestance - gives +50 CHA, +7.5% aggro on all melee actions, requires 150 STR & Veteran Squire") put XP in them, ding, you have a new skill (passive or active). You can go deep to be a high-end tank, or wide to adjust to groups ("ok, I'll heal, let me reorganize my skill bar").
- No quests, or very few long-term quests, meaning you can do whatever, including going to help guildmates at the other end of the world and not lose out on XP/gear/gold/rep/whatever.
Example: you don't have quests at all. The game simply displays your 5 most advanced achievements as a suggestion of what to do ("Dungeon Janitor - kill 37/50 different dungeon bosses" "Have Jboots, will Travel - discover 28/35 Inns"), but you can ignore some if you really don't care about them.
- No instances, a very large (procedurally generated?) world
- Multiple interlocking factions with progression/regression and zero barriers to interacting with other players
- Multiple paths to gear acquisition (crafting, random gear drop, named loot tables, AH, faction vendors, etc)