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"If the class/level/quest model is so bad then answer me why Tabletop gamers (Dungeons and Dragons etc.) have been using it for over 30 years. They've had more than enough time and freedom to change it if they wanted to - why haven't they?"
Two main reasons, one of which applies to CRPGs, one of which doesn't.
1.) It's traditional. It's been around this long, if they introduced a sweeping, fundamental change to the system, they would lose a huge percentage of their playerbase.
2.) It's simple. Relatively. Especially now, with 3.5 Edition. No more THAC0, streamlined rules, mostly d20 rolls. No one wants to do excessive amounts of calculations (with notable exceptions being kicked out of the group) while playing D&D, they want to have fun. This problem doesn't apply to CRPGs. Calculation can happen in the background, billions of times a second, without the player even needing to acknowledge their existence.
In essence, CRPGs use the familiar level-based advancement system because it is just that: familiar. There's nothing preventing them (developers) from exploring new mechanics. It's merely a tried and true method.
Listen Asmodeeus, seven years ago, Ultima Online didn't even have those pathetic "quests" that you refer to or those "professions" of ninja, samurai, necromancer, and paladin. Nor did it have any of the neon crap, or bug mounts. It didn't even have any "combat moves." You turned on attack and jousted with simplistic swings. It was a better game then. if you can't guess why then just uninstall the thing and move along. - Crabby