I've never tried any Final Fantasy; they strike me as not being very "deep", more for children than adults, but I could well be wrong.Depends. The main sequence (up to 9 anyway - haven't played past that) don't have much depth in the core character advancement system - gaining a level just gains some small automatic stat increases of vague significance. Most of the games provide (if not require) additional means of improving a character, however, usually having something to do with how you learn magic spells. Final Fantasy VIII took this to an extreme, rendering actual character level nearly irrelevant with its munchkin's paradise junctioning system. (Even so, it's still not exactly deep - more like "grossly imbalanced".) The Final Fantasy Tactics line, however, provides some satisfying complexity in character advancement and customization, not to mention combat (as the "Tactics" in the title would suggest).
As for actual roleplaying, they pretty much have none - main characters and their dialog are all predetermined after FF1. The first Tactics game allows you to hire additional units which you may choose the name and gender of, and FF1 allowed you to create your entire party from scratch, but in both games these characters were just blank slates - the only personality they would ever have is what you imagined for them (though that could still be fun). Story varies in depth and quality from game to game, but if a story is ever bad (Tactics Advance >_<) it's never because it was "for children" (though in and before the SNES era many games were vigorously mangled in translation to make them "kid friendly" - if you intend to play games from that time, I suggest finding newer translations). Some of the stories have interesting philosophical and moral ideas behind them (the nature of the afterlife - and life itself, to a degree - is particularly prominent in 7 and 9). Most can manage to be at least entertaining. And since it's in a similar vein, do not forget Chrono Trigger (forget Final Fantasy before you forget Chrono Trigger), an epic myth that easily belongs alongside such other classics as The Odyssey, Dante's Inferno, Star Wars, and Planescape: Torment.
Fallout/2 has an excellent combat and stat system, and the first has a pretty good if comic-book-style-over-the-top story (Fallout 2's story is pretty goofy, though). In both, roleplaying is somewhat limited and leans strongly in the "smart-ass" direction. The main draw of Fallout/2 is the setting, though; a veritable character in itself.
Aside from another great setting, Arcanum is noteworthy in that you truly start out as a nobody thrown into things of astounding importance purely by chance (alright, and maybe a pinch of destiny, but just a pinch). And consistent with starting out as a nobody, it is entirely possible to start out utterly incompetent, as well. Want to be a bungling fool more likely to injure herself with a sword than her enemy, and whose knowledge of magic begins and ends at the words "abra kadabra"? Well, you can. And you can then build that bungling fool into a great hero, steeled by her experiences to become master of sword and spell both. (Of course, you can start out competent, too, if you want to, but it's not nearly as fun :))
Wizardry 8 suits your combat needs, I think. Certainly challenging, with a moderate approach to the use and power of magic (more for buffing and crowd control than killing things) that may or may not fit what you mean by "low-power". The story is good, though there's not a lot of it, and nearly all of it is concentrated in the last few minutes of the game. A good number of unique, interesting characters all with great voice acting partly make up for this, but the game is still mostly about providing a smцrgеsbord of turn-based combat and classic dungeon crawling. Roleplay is nearly non-existent, your characters being mostly blank slates except for a rough personality determined by which voice sets you chose for them. Maturity doesn't really come up, except perhaps in character design - female NPCs all have sexual characteristics exaggerated to the point of just plain not looking human (though simply bad character modelling may play a part in this), and the only female NPC that fights is also the least-clothed (in metal lingerie, of course).
Jade Empire has a very...familiar story. You Are The Only Hope in a battle of Clear-Cut Good Versus Evil for The Fate of the World(/Galaxy/Kingdom/Republic/Empire/etc.). The sort of story that's been done a thousand times before. However, Jade Empire does it really, extremely well. And in so doing serves as an example for why so many stories like these are told - because when they work, they feel really damn good. Having one of the best musical scores ever only intensifies the experience.
Some complain Jade Empire is on the short side, but that wasn't my experience; my first game clocked at 49 hours, my second at 30, and honestly who expects an action RPG to be as long as Baldur's Gate, anyway? My only real gripe with the game is in roleplaying: there's minimal ability to customize your character's appearance, and far too many times (as in any at all, but more specifically: more times than I could count on my hands) where you are limited to just two responses in dialog to choose from, frequently at opposite extremes of the game's moral axis. Even more frustrating, these railroading dialogs seemed to crop up MOST often at important, emotional, plot-vital, and all around character-defining moments - y'know, moments where the player should be given the largest number of dialog options with which to differentiate zir character.
Oh, and wait, that's not my only gripe, actually. There's also the way Bioware shrank away like a bunch of babies from their own content, too cowardly to show the same full kiss scene for same-sex romance climaxes that they do for opposite-sex ones (though I fixed this (
http://www.lucasforums.com/showpost.php?p=2401203&postcount=173)).
Wow, many of the older games are ridiculously priced!
Planescape: Torment, Daggerfall, Arcanum and Wizardry 8 are all around $50 each. That's quite pricey for old games.That's why you download them. Copyright holders don't get money from second-hand sales, anyway.