As anyone who's played World of Warcraft can attest, a sufficiently satisfying system of levelling up can make a game playable even in the absence of plot, depth of gameplay, or stimulating action.
Disgaea: Hour of Darkness for the PlayStation 2 takes this concept and runs with it about as far as it's possible to run. And then it goes back and adds in the plot and depth of gameplay anyway, just for kicks.
In Disgaea you'll find yourself taking on the role of Laharl, a spoiled young demon brat. Laharl's the son of King Krichevskoy, Overlord of the Netherworld. For initially unexplained reasons, Laharl's been asleep for a little over two years, and when he wakes up he discovers his dad's dead, the various demon lords are all vying for the title of Overlord, and pretty much everyone's forgotten that Laharl exists.
Naturally this situation won't do, and Laharl proceeds to mobilise his vassals, including the backtalking demon wench Etna, and a squad of demon-penguin-thing Prinnies. Together they set out to take back the Netherworld and install Laharl as its ruler.
Of course, things get complicated along the way. Laharl and Etna get mixed up with a ditzy angel named Flonne, a bunch of humans, and a world-spanning plot of conquest, and what begins as internal squabbles in the Netherworld soon threatens to spill over into both Earth and the heaven-like Celestia.
The way all this plot works out as gameplay is via a series of turn-based strategy battles. You deploy up to nine characters into a map, and then proceed to mix it up with a bunch of enemies, until only one side is left standing. It's fairly routine turn-based strategy fare, but with a bunch of tweaks which make this one of the best in the genre.
Some of the tweaks are little things. The notoriously slow pace of most turn-based strategy games is notably faster here, thanks to enemies which all move at the same time, and a well-tuned user interface.
You have the ability to lift and throw both allies and enemies, which makes for some interesting tactical options. Also, some maps are littered with "geosymbols", which bestow bonuses or penalties for standing on certain squares. Geosymbols can be moved and destroyed, and when destroyed they cause explosive chain reactions throughout the level which can be manipulated to create huge combos and destroy many enemies.
The real genius of Disgaea, though, happens between levels, and it's mostly in the levelling up. Each and every character in the game can level up to level 9999. You'll only need to get to about 80 to complete the main plot, though. But the game doesn't stop at the main plot. There are additional challenges, up to and including a final bonus boss which you'll need at least level 6000 to take on.
It's not just the characters that level up. Each skill you learn can level up with practice. Higher level skills do more damage or strike a larger area of effect. Each item, weapon, or armour can also be levelled up to level 100, increasing its stats. You level weapons by entering an "item world", which is a randomly generated dungeon of a difficulty matching the quality of the weapon. Each level conquered raises your weapon by 1.
Within the item worlds you'll find "specialists", which are specific monsters you can kill. Killing a specialist grants a further bonus to a item, and subdued specialists can be moved from one item to another and stacked on top of each other.
To make things even more complex, characters can assume one of a number of jobs, ranging from various varieties of magic user to archers, samurai, and ninjas. The jobs themselves level up with practice. Characters have the option to "transmigrate" to a new class or an improved version of their existing class. This involves starting over at level 1, but retaining the skills the character already knows, and gaining a faster rate of stat gain based on how strong the character was before transmigrating.
Mixed in with all this is the Dark Assembly, a kind of Parliament of Hell. Here you can propose bills for the Assembly to pass, which can have the effect of making higher quality gear available at the shop, raising or lowering the level (at XP benefit) of enemies, improving a character's abilities, or opening additional bonus maps. Getting the Assembly to pass your bill will require bribing senators with items, or alternatively laying the smack down on any senator who dares to vote against you.
The Dark Assembly can also give you the ability to create new characters. Characters created in this fashion become the "pupil" of an existing character. Characters mentoring a pupil have the opportunity to learn skills obtained by the pupil, meaning you'll likely set up some long mentor/pupil chains in order to equip the majority of your characters with offensive and healing magic.
The sum of all this makes for a fantastically addictive experience. You'll be creating hordes of characters, levelling them, trading skills between characters, improving items, and transmigrating. The game does a good job of making every increase in power feel meaningful, and setting worthwhile challenges so that all that levelling doesn't go to waste.
The basic gameplay is very solid, and becomes moreso once you master the complexities of the metagame. The plot itself is very funny, lighthearted, and entertaining, with some over-the-top but well delivered voice acting for the main characters. The game has a variety of alternative endings, and you can carry over your characters into new games complete with their levels, skills, and items, so there's plenty of replay value. You can easily spend a good 80 hours just making your way through this game the first time, and a good deal more on subsequent trips.
The graphics are fairly basic, mostly consisting of sprites, and probably could have been rendered on a GameBoy Advance. But they're still not hard on the eyes, and are totally appropriate given the setting and gameplay. The sound is nothing particularly noteworthy; as mentioned, the voice acting is good, both in the plot scenes and in the various shouts and cries your characters utter in battle. The music is sufficiently interesting to not having you tearing out your hair after 80 hours, but otherwise not particularly memorable. The sound effects are a satisfyingly beefy mix of spell effects, explosions, and weapon-connecting noises, and are all par for the course in this sort of game.
It's really quite hard to pick faults in this game. The most obvious thing to say is that if turn-based strategy and levelling up are just fundamentally not your thing then Disgaea really has nothing to offer you. It doesn't make any pretensions at trying to reach a large audience - it's focused tightly on its core mechanics and sticks to them. You won't find minigames, social play, or a lot of cosmetic customisation going on, let alone meaningful choices in how the plot plays out. Disgaea is about levelling up and killing things, and that's all.
Also, Disgaea suffers from the bane of the Final Fantasy series, in that a lot of the high level attacks and spells have annoyingly long animations. Often you'll be able to put down the controller and have a sip of your drink while waiting for your characters to finish their absurdly lengthy action sequences. If you've been able to stomach this in other roleplaying games, then you won't have a problem here, as Disgaea's aren't quite so bad as the worst offenders in the genre.
These issues aside, there's really not a lot wrong with Disgaea. It's an almost perfect example of the sort of game that it's trying to be, and it's hard to criticise without criticising the genre generally. It's likely from this post that you've already got a fair idea whether this is intrinsically the sort of game you hate, and if it isn't, then you owe it to yourself to play.
Disgaea is quite hard to get now in a lot of countries. To the best of my knowledge it was never even released in Australia. I have a UK import copy, which I understand is rare even in the UK. I'm not sure what the situation is in the US, but seeing as the sequel has recently come out over there I'm hopeful that the game's more common. In any case, if you see a copy, get it, as you may not get another chance.