Yes it took years to find out what I truly wanted from MMORPGs but I have finally found out what it is. It had eluded me for so long that I began to stop caring about what I played so long as I had even the slightest bit of fun. As it turns out mere tidbits of fun were simply not enough. I have quit many MMOs because I stopped having this "fun" but most of the time I was left wondering "Why?", and I'll tell you why.
It wasn't bad mechanics, terrible communities, lack of character customization or any other bad quality the games may have had. It was probably the fact that the game didn't have what I as a player was looking for and so desperately needed.
So the years passed and I took on game after game, trying to enjoy them yet quitting as quickly as a single hour of playing. The games just totally lacked what I had to have to make them absolutely godly. I needed a truly living environment where things I did alone made a difference no matter how slow and insignificant. I needed the ability to live in such a world but not have to socialize with anyone if I didn't want to. I had to be able to log on and know that no matter what I did I could have fun doing it.
What I needed was a game that offered a single-player experience in a multiplayer world. I don't like grouping. I find it tedious and unnecessary for many tasks. From a roleplay perspective, my characters have never gone after "great evils" or giant dragons. They just do what they want to do. For example: I play as a lone vampiric daemon who is cursed with demonic blood and can walk in the daylight. He doesn't drink blood but drains soul essence and can only do so from other vampires. He doesn't need to eat often, but when he does it requires a hefty amount of casualties.
His days are rather simple. They involve hunting the "evil" vampires, evading humans who would see him dead and offering his mercenary services to various lycanthrope tribes in the area. He travels a lot, but doesn't necessarily explore in the typical sense. He leaves a spot knowing he'll find another good spot to camp out at or call home for a period of time.
No MMO I know of offers such a personalized experience, especially for someone who enjoys playing as a vampire. The same can be said for most of my other characters except for my warriors or paladins, though they DO enjoy exploring and never know where they will end up. The only game that comes close to this so far has surprisingly been Age of Conan. While I developed new characters for the game's lore, I found it offered a delightful solo-player experience that didn't force you to group up and offered a good bit of exploration. It still feels like a game though rather than a world and I can't be anyone I want to be.
Second Life was actually one of the best spots I found for freeform RP, but it lacked built in mechanics to satisfy the typical MMO gamer. Everything either costs money or needs extensive scripting and I found myself quickly NOT enjoying roleplay in SL. In fact, I stay far away from anything but the military scene. Again though, even the military stuff lacks personalization as you have to conform to a specific role (no matter what you happen to be playing as).
In terms of exploration only one MMO has really given me that and it was SWG. I found myself exploring vast regions on a speeder and taking ages to get to my point of interest. It was hard to roleplay in the SW universe though as my tastes are more sci-fantasy and require magic and swords, and maybe guns. Hell, Fallout 3 seemed to have most of what I needed in a game but lacked social aspects (and vehicles).
In short, I know now what I need to keep me hooked for a long time.
Single-player experience with fully voiced actors like that of Tortage in AoC.
Skill-based progression like Oblivion, Fallout 3 or even Darkfall.
Vast uncharted and explorable areas (not exactly seen in any MMO except perhaps EVE).
A bit of social content, just enough to make you feel like you're living with other people.
A living, breathing world with choices, consequences, rewards and punishments.
I think that perhaps a Fallout or Elder Scrolls MMO would satisfy my needs but those are a long time coming.