FF Tactics/FF12/Vagrant Story online (preferrably not operated by Square Enix - just allowing another company to use and expand upon their property) Strategic gameplay is lacking in most of the MMOs I've played, and it's the key element in this branch of the FF franchise. An updated action-y turn-based combat system and body-part targeting would be interesting.

Xenogears (not Xenosaga) with a mix of on-foot and mech combat, even allowing mixed battles. I especially liked how "magic" was handled by technology through the "Ether Drive"... since I hate traditional fantasy magic and real magic. (the only big issue is the special nature of the "good" mechs responding only to a set group of animas, although there were some pretty strong unrelated gears that all players could potentially use.)

Metroid - exploring the universe and hunting aliens in a cybernetic power suit that can shrink down into a ball. Going after bounties and finding powerups as you go. Mixing weapon properties on the fly. Oh yeah, and a personal ship as a customizable home with trophies. Fun times.

....

3 is good enough for 1am.

My "Big 3" criteria:

1 : PvP:
     Either it shouldn't exist, or must be by choice, never free for all. It can be contained in zones that have zero interest to PvE players, or be only initiated by duels (with an auto-ignorable challenge system) or realm wars where people expect it (still separated from PvE players).  Where PvP exists, it should be a friendly competition, ideally.  No permanent damage should result to either party involved.
     I should never have to be looking over my shoulder while minding my own business.

PVP style is a deal-breaker, regardless of anything else the game has to offer.

2 : Classes / Combat options (gameplay):
     I prefer physical combat, but this can be expanded upon in a sci-fi setting. I'll never touch mage jobs, and there has to be the option of not taking any magical abilities without harming the overall effectiveness of a character. I prefer unique weapons, not your standard medieval weapons or laser guns, no matter what the setting is.
     Also, there has to be a lot to do during combat, not just auto-attack and unleash a special attack every 1 to 5min. The more there is to do, the more potential there is for strategy, as long as it's not just button mashing to get the same results.

3 : Solo-ability:
     My schedule is not my own. I cannot generally lock myself into being in a game at a certain time for any period of time. I get called away. I want to be able to accomplish most things on my own, with group-needed events taking under 2 hours (including gather time).

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Those are my "Big 3" criteria. If those are missing, I won't be able to enjoy the game for long (or at all in some cases.)

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These next things may draw me to a game, or hold me in the game longer than I intend to stay:

4 : Friends:
     If I know people who are playing from outside the game, I'll join/stick around to help them out. I don't go out of my way to make new friends. I can also happily play MMO games without making any friends (I HATE when games force you to add a certain number of people to your "friends list" as part of the quest lines.)

5 : Story:
     I like playing through quests when they're part of a larger story that you complete progressively through the game. A story can justify "collect 10 rat tails" type quests, especially when semi-cinematic cut-scenes are involved with the story before and after.  I'll continue playing long after my friends leave a game if there's more story to be seen, even if that story can be watched on youtube (it's not the same).

6 : Price:
     I only play 1 P2P game at a time currently, while I play several F2P games.  Point 4 is drawing me to other P2P games right now, but I can't justify the expense yet.  Free to play games offer variety from the P2P game experience that can get boring at times.

7 : Developer Feedback:
     MMOs are constantly changing and growing.  The developers should have a clear goal for their game, and each thing they add to the game should build on that goal in some way.  The players should understand that goal.  Each time the game is changed in some way, or something is added, the developers should say why.
     With this understanding, players can offer more constructive feedback and suggestions.  People who want things that go against the goals of the developers can easily be discarded and be rightly told to go play another game with that goal.  A person's subscription fee does not entitle them to creative direction of the game.
     When developers fold to the whims of a vocal community, when it goes against the game's goals, that's when you have problems, in my opinion.
     Sometimes the vocal community can be right, and are trying to get the game to stay true to its goals in a significant way that the developers either are already working on, or may never have thought of before.  In these cases, it would be of monumental importance for the developers to acknowledge the issue and say what they're doing about it, even if it's just "we're working on it."  Even better, would be to discuss the options they're thinking about, and open it to discussion, so what they finally do end up implementing is what people actually want.

     If developers are aware of the state of the game, and show a real understanding, I have hope that the game may change for the better.  This shows there is potential for greatness.

     When developers are silent, and simply release updates without their reasoning, players can feel like they are being ignored (and they very well may be.)  This problem becomes even more noticable when there is a language difference between developers and players.

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There are a few other things I look for, but they don't make or break a decision.  They would read more like a wish-list, so I'll save that for a relevant topic.