What does it take to make me play a game again? That's actually a complicated question, because I've been making more of an effort to pick through the backlog in recent years. Once upon a time, all I needed was to have a good time with the game in the first place to play it again. Shining the Holy Ark and Albert Odyssey: Legend of Eldean are fun games but don't have much to prompt a replay (I realize that using Saturn RPGs as my examples instantly excludes most of my readers, sorry about that). Nevertheless, I replayed both of these more than once because I had a blast the first and second time through. Having a good time is no longer enough justification to replay a game for me, though. There have to be other reasons.
Optional content is all over the place in RPGs from recent years, and it can conceivably prompt a replay. Super Robot Taisen: Original Generation games have sucked hundreds of hours from me with their branching paths, optional units that are fun to unlock, and the free hand in character and unit development that lets me play them differently each time. Not being able to see everything the first time through is an excellent goad to playing a game again; I haven't done so yet, but Front Mission 1st will most definitely be getting a replay so I can see the second storyline. The first English Fire Emblem offered Hector's storyline and the Hard mode for Eliwood and Hector to go through, which I took advantage of. Shining Force III has a couple of characters who cannot be recruited together - one character can join, the other will fight the player and die. Summon Night: Twin Age opened up a lot of extra areas to explore after the main story was complete, and while I had to move on to something else at the time instead of playing much past the ending, these additional portions certainly seemed worth looking into.
There are, of course, other means of inducing replay. I don't play much online, and the few games I play that do have online functionality make no difference to me -- I don't use it. Unlockable outfits and costumes also have no effect upon me, though if I had a nicer TV that might change. Luminous Arc 2 has a lot of character interactions that require multiple plays to experience, but the game as a whole didn't work well enough for me to want to investigate this. Shadowrun on the Sega Genesis would be a very different experience if I played it as a Decker or a Shaman, but once was enough for me to feel satisfied.
The moral of the story seems to be that unless the game is good enough for me to want to keep playing after I finish, whatever the developers stick in as an incentive to do so isn't enough. That may not be a particularly deep or insightful statement, but mediocre games that queue for replay keep appearing, so obviously the lesson hasn't been fully learned.