Personally, the difference is the IP in use and how well the launch is handled. Warhammer had a pretty decent turnout...until the mess that was PvP reared its head in. Broken PvP in a PvP-centered game is disastrous. Then we have stuff like Age of Conan, which unfortunately only appealed to the people who bothered to read the books, as that Conan is way different from the one used in the movie or TV series. And the brokeness of its launch was pretty bad on its own. Same with LotRO, though it lasted longer because Return of the King was still fresh in people's minds.
TOR has outlasted them all, but it was to be expected between the insane amounts of money Bioware spent advertising the game, the power of the Star Wars IP and the fact Bioware was behind the game. Then the stupid design changes and other problems broke out.
I personally think FFXIV will be fine so long as they stay true to their design, take advantage of the power behind the FF IP, make changes that don't piss too many people off and continue to come clean and transparent as they have since Yoshida took over. Because shadiness and BS is much more likely to drive people away than most would think.
As for me:
Everquest (1 year)
Lineage II (2+ years)
Ragnarok Online (1 year)
City of Heroes (2 years)
FFXI (5 years)
WoW (5 years)
Assorted titles: Granado Espada, Champions Online, Warhammer Online, TOR (beta only)
My main thing is game mechanics, because well-designed game mechanics ensure that shenannigans like FFXI's party dynamics don't happen. Story and all goes to the wind when the gameplay sucks. Progression models come in second, not for my sake but more to prevent seeing people do stupid things to each other in the name of efficacy (again, FFXI being the posterchild for that brand of stupidity). Then comes lore, setting and story, because lore that makes sense is always something that should be present in-game.
