For me it's when the game continues to present many goals to pursue, and hopefully those goals have tremendous variety.  Here's a small checklist of goals that have held or would hold my attention for considerable time in an MMO: 

• Progression raiding when there is still content to progress through. Although I haven't played an MMO where I've dedicated time to a raid schedule in over 3 years now, I can't deny how important this was in keeping me around in my hayday.  Competing against other guilds for server-firsts was incredibly rewarding.  Once all the hardest content was conquered, farming the raids for new gear would remain gratifying for a few weeks; but most of the satisfaction (for me) came from the challenge and competition. 
• As a subset to my previous point, gear progression.  Lottery-style random loot from difficult encounters held my attention a lot in the begging of my MMO career, but I feel it's dissipated over time.  The days of DKP and getting 1 new upgrade every 4 months were exhilirating, but undoubtedly would have gotten stale over the years. Since then, gear progression in most games has gone on steroids and has lost most of its luster to me.  
• Collectibles with persistent in game relevance.  I grew up catching 'em all on the gameboy.  As such, I was obviously happy to hear about WoW's whatever-it's-called pet combat system.  I haven't played the game since it's been out, but I know I'd spend a ton of time finally collecting those little monsters that I never gave 2 shits about before.  Rift's Dimensions system is also pretty incredible in this regard, offering a constant gold sink with seemingly infinite rewards and infinite possibilities.  
• As a subset to the above point again, player creations.  When you allow players to create stuff within the game world, like the housing in Rift, the replayability can feel like it lasts forever.
• "Horizontal progression." A game like FFXI had you pursuing sometimes dozens of progression goals at any given time.  From unlocking subjobs by doing their lengthy quests, to your nation's missions, to limit breaks, to unlocking keys to areas, airships and chocobos.  Most of the things you pursued in FFXI (when I played in vanilla through the first 2 xpacs) were not gear and stats.  That was there, to be sure, but there were so many other important goals that gear just took a backseat.  These things kept me playing for longer than the game probably deserved.
• Friends.  Can't ignore those guys.  They tend to be the reason you find yourself just jumping around town for months after you would have preferred to stop playing.    

#1-9: Guild

#10: good two way developer communication.

This is why i'm going to play Neverwinter. The Foundry alone offers endless gameplay.

Originally posted by Allacore69

This is why i'm going to play Neverwinter. The Foundry alone offers endless gameplay.

Endless does not necessarily equate fun. I would much rather have a higher quality game that ends earlier, than a non-fun one dragging out.

Having said that, i have nothing against foundry. If it produces even one good module, it is gravy. At worst, i will just ignore it.

My ultimate objective in a mmo is to dominate the virtual world. In the meantime, i enjoy doing friends (and enemys) in the proccess and having much fun with them.

Originally posted by Cephus404
I don't play end-game.  Once my  character hits max-level, they get retired and I either start a new character or I go play a different game.

It's sad to say, but I am the same way unless there is an awesome crafting system to keep me interested. But, there is nothing out right now that can keep me playing it.

WoW burned me out with the rep. and gear grind so much, that I can never do it again.