The MMOs that have lasted me the longest are...
(no particular order)
Asheron's Call (first MMO, 1999-2001)
Ultima Online (about a year, then they changed it and added neon colored mounts...)
Star Wars Galaxies (for a few months, but I got in at the wrong time, because when I was really getting into it they changed it)
Eve Online (a few months again, might give it another shot)
Now lets look at the non-sandbox MMOs (no particular order)
Everquest: a few days
Everquest 2: A week
Asheron's Call 2 (a couple weeks)
City of Heroes (a couple weeks)
Guild Wars (less than a week)
Lord of the Rings Online (2 weeks)
Age of Conan (a week or two)
Tabula Rasa (a week, plus a few days of beta)
(and I might be missing some, but those are ones I found that I played when searching through the game forum section to the left)
Now sort of an exception is world of Warcraft, that I played for 2 to 3 years (off and on), but every time I made it to "end game" (around level 50 before expansions, then around level 55-60) I got incredibly bored, and remade my character. I really only played it since my friends played it and kept wanting me to play, and my cousins played it and wanted me to play...so yeah. But I never truly had fun with it like I have the sandbox MMOs. Surprisingly I lasted a while though.
So looking at my list...looks like overall, the sandbox MMOs (at least for me) have lasted the longest in game time played. And I was trying to think of why...and the only answers I could think of were:
An open world
generally have awesome PvP that isn't instanced (instanced PvP, lamest idea ever...woo I'm going to play capture the flag! lol)
Housing or even player cities or stations (non sandbox MMOs tend to lack this)
Skill based instead of class based (class based, I can NEVER pick a class that I like. So I always make lots of alts. Skill based gives more freedom too imo)
And there is a risk vs reward usually...for UO before they changed it, I really had to be careful otherwise I would lose all my items on me. If there is just a reward and no risk, then it isn't really exciting or fun. eve Online if you aren't careful, lose your ship and all the modules on it. In themepark MMOs, I could not care less if I died...I'll just kill my character to teleport to the nearest town. No risk at all. In WoW I have to repair or get rez sickness...I never cared about that though, so I never cared if I died, there wasn't a lot of risk. And I didnt even care if I died in PvP...it was boring, never exciting like it was in UO or Eve Online. It felt like I was playing an FPS where I respawn and it doesnt really matter if I died or not.
Well that about sums up this.