Late response! But finally got around to finishing one, so here it is.
While this has varied over the years, I increasingly find myself playing for gameplay.
First of all, I should start by saying that an RPG is fundamentally a medium that sells itself on a lot of things at once. It can show off pretty graphics very well (whatever you think of the FF series, there's no denying that it is one of the franchises near the fore of showing off brilliant computer graphics work). It is a vehicle for strong soundtracks, due to the length and scope of the games, and the variety of situations the games portray. It can tell a story better than most game genres, with a larger, more detailed cast of characters, and create strong settings. It can have interesting battles, and it can have interesting ways of setting up to beat battles ahead of time.
That said, while getting the full mix is nice - and occasionally an RPG will surprise me in one category (see Suikoden 5 having a good cast and storyline) - gameplay is the one thing that keeps me interested after fifty plus games. I mean, if you think about the rest...
-Story, characters, setting: First of all, problem number 1 is that all these things are done noticeably better in other genres (books in particular). So if this is your reason for playing, I'd suggest taking up reading instead - it's cheaper, more portable, and builds your language skills, quite apart from the step up in quality. Additionally, though, there's this problem with RPG plots recycling the same anime tropes over and over again, and I for one found it very tiring. The formula was interesting enough the first dozen or so times, and in particular the first few times I saw it done -well-. It is not interesting any more to me. I'd say there's really only ~15% of RPG plots/etc. left I really care for enough to give out significant points for any more. In the past... two years, Suikoden 5 is the only game I'd score significantly higher for plot than I would if the plot hadn't existed at all? XS3 might have done the same, but it also managed to become the only game I have ever lowered for plot concerns as well. To continue that thought, mediocre plots rarely annoy me too much. Likely due to considering the vast majority of RPG plots mediocre anyway, the ones that are "more" mediocre don't really ping my radar. Sure, I can't defend MEGA MAN PLOT to my litereature prof, but I can't defend Lunar plot either.
Graphics - I've never put a large amount of emphasis on stunning computer graphics. Anyway, only a few games are notable at this. I guess I could see giving something like FFX points for this, because it does make environments more immersive... but even that's a stretch, and anything below that? Yeah, who cares. General aesthetics appeals to me a bit more, but if I want beautiful art, even beautiful art in the anime style, I can find it without shelling out for a game, or spending forty hours on it. Like plot, very beautiful art can add to a game, but only so much, and I wouldn't tear down a game for mediocre art.
Music - After gameplay, I think this is the one I have the most trouble arguing against as actually important to me these days, because what RPGs do with music is actually somewhat notable. In particular: the music is something you are supposed to respond to (e.g. a tense boss music gets you in the mood for a good fight), which is something also seen in movies/TV, and conversely, the music responds to you (it syncs up with whatever you're doing at the time). It's really kind of neat. The overall quality isn't the absolute best you'll find anywhere, but due to it being generally more my style than most popular music, it tends to be good enough for me.
That said, a game is not its soundtrack. If it were, once again, I could just download the soundtrack separately from the game and get most, if not the entire, experience. And playing an RPG on mute shouldn't instantly take off major points from it, one would hope. It's probably not unfair to dismiss an RPG's music as "mere aesthetics", even if harsh.
So that really leaves gameplay. This is the one thing RPGs have that can NOT be reproduced in other genres, let alone bettered. RPG gameplay is pretty unique. Fundamentally, the strategy of it does appeal to me - you have to think out the best plan of action to get out of a situation, rather than rely on twitchy actions to get you through things (not that I dislike games that do the latter, but the point is RPGs are different). There are a few other genres that do this, but RPGs interest me most, because they have something ELSE in their gameplay that appeals to me a lot, and that's their party dynamic.
When I'm playing Megaman, I have to decide how best to control Megaman to get me past the current challenge. When I'm playing MGS, I have to decide how best to control Snake. When I'm playing Wild ARMs, though? I have to decide how best to control Rudy, Cecilia, AND Jack. This really introduces a whole new way of looking at gameplay for me, and the interplay is something I just fundamentally find interesting. It's also an interplay nearly unique to the RPG genre - more action-oriented genres only really allow the control of one PC, typically, while strategy games will have me in control of many more (typically generic, copies of each other, which causes me to be relatively less interested, not that there aren't some strategy games I enjoy). Actually, this is part of the reason ARPGs typically score far below other RPGs to me; the emphasis is on the control of ONE character, rather than the many. (Similarly, why VP was able to far outperform other ARPGs in my estimation, if it counts as one.)
So RPG gameplay has two things I like a lot, and I really enjoy seeing how different games will put a new spin on it. Fundamentally, that's what I look for in an RPG these days. Can it deliver good RPG gameplay? Does it force me to think of ways past RPG battles in new ways? Or set up for them, perhaps... are there a variety of ways to assemble your party to achieve success that can be considered?
Looking through my top games, a few can easily be seen to completely deliver in this department. Without further elaboration on why, they are: Final Fantasy Tactics, Final Fantasy X, various Fire Emblems, Xenosaga 2, Mega Man X Command Mission, Wild Arms 4, and Grandia 3. Some of these games even deliver strong stories as well by RPG standards. In general, the more they deliver in other areas, the higher their score, which is proof that at least, yes, other factors do matter. Every one of them produced gameplay I absolutely adore, keeping me constantly interested in battles of all types throughout the game.
That leaves the following games which don't have incredibly awesome gameplay but I still like. So the question is, why?
Final Fantasy 7, Suikoden 5, Xenogears: Gameplaywise, these are the three weakest games that I put at 8/10 or higher. So why are they here? For being in that rare elite that actually have notably, really good stories to me. Note that 8/10 is as high as they get; beyond that, gameplay is a must.
Valkyrie Profile: The gameplay is decent if a bit repetitive. What this game really has is creativity, something I highly prize. The gameplay itself has some of the creativity, but where the game really shines is in its setting, story execution, etc. RPGs are, as mentioned, in general such a massively stale job for storytelling, that when a game tells a very different story, and even does it pretty well... that's going to mean lots of points.
Suikoden 3: The gameplay is merely good. Pair system is neat, skills system helps differentiate a huge cast. Otherwise? Same ballpark as the above three big plot games, with trinity sight system and the epilogue PoV meaning extra rewards for the creativity I love.
Final Fantasy 6, Chrono Trigger: Two games that occasionally bother me for being this high because I fear they're tainted somewhat for nostalgia. But there's more to it. FF6... well, while actual battles are merely decent, it does manage to score very high in gameplay for having a very neat character setup system, being a perfect balance (to me) between strong base templates and plenty of things for you to mess with on top of that, with the equipment system in particular being especially interesting. It also has one of the best and gutsiest midgame plot twists in an RPG, something the genre seems very reluctant to do. Chrono Trigger... well. I dunno. I think it's mostly just really highly polished. I can't think of much wrong with it, even if it's not too interesting, and it's just so... playable. A tough one to pin down.
Wild Arms 3: Decent battle system, brilliant character setup system (highly customisable but the base PCs make a huge difference), excellent storyline, brilliant soundtrack. Despite not being top five for gameplay for me, it's just way too good at like everything else, beyond occasional WA stupidity with puzzles and world map.
Breath of Fire 4: Odd one. Battles are good (manipulatable turn-based, combos, etc.), but good enough for an 8? Probably not. Master system is good but doesn't floor me. Soundtrack is good, Fou-Lu is love... yeah, it just adds up.
Phantasy Star 4: Similar to BoF4 in many ways, but without the soundtrack or Fou-Lu. So why's it so high? Actually, I'm not fully sure. It's not a bad mix by any means to start with, but... hmm. The game probably benefits unfairly from its awesome out-of-game documentation, and beyond that, it's just... Chrono Triggerish, a joy to play through.
Pokemon Emerald: Well, Pokemon gameplay in its own way is pretty hot, so this one being where it is probably isn't much of a mystery. This one in particular cracks the ranks of the elite because its boss battles are just about my favourite in any RPG ever.
Mm. Closing thoughts... another thing I care for a fair deal in RPGs is pacing. I love it when RPGs keep me constantly interested. Sure, the best way to do this is a good battle system, strong plot, etc., but it's even better when you can string things along in a way that just wants me to keep playing, with the game firing on all cylinders at all times. Wild Arms 4 is exemplary at this; Phantasy Star 4, thinking on it, is also pretty strong at it. Having a shorter length often helps a lot, because this means less filler, but on the other hand, if I'm really loving a game, I'll want it to be longer, and adore it when the game actually delivers on this (Suikoden 3 gets away with it the best, perhaps because of the rotating perspectives keeping things from becoming stale).
Neat topic, Rob.