The Shin Megami Tensei (or MegaTen) series is pretty huge in Japan. I've often seen the claim repeated that it is the third best-selling RPG series in Japanese history, behind only Dragon Quest and Final Fantasy. Knowing only that, you'd wonder why Atlus never translated any of these games into English and tested the waters with them over here. The reason is that Nintendo of America, in accordance with their censorship policies of the time, would have had to cut the game so drastically there probably wouldn't be much game left to play.
Shin Megami Tensei takes place in Tokyo only a decade or so from now. What's basically going on is that there's a war brewing up between the followers of Law, headed up by the Christian God, and the followers of Chaos, led by Mr. Lou Cypher himself. A scientist of the Chaos side has started summoning these demons from the Abyss as part of some complicated plot to revive the Devil. Unlike in most games, where conventional morality would slap you firmly on the side of the Holy and send you out on a monster slaying quest, the game lets you choose what side you want to support - if any at all. The actual "proper" path through the game is to remain independent, and slaughter both sides because they're both basically a bunch of jerks.
The Law side wants to establish something called the Thousand Year Kingdom, which would bring peace and happiness and etc to the human race - well, the ones that bow down to Yahweh and follow his strict rules, anyway. Everyone else will be killed. In fact, the Law side has decided that Tokyo is too out of control and that the only way to build their Kingdom there is to start fresh, so they're going to lob a cluster of nuclear missiles at it and blow away everyone, both demons and innocents. Meanwhile, Chaos wants a world where no one is compelled to do anything, but then there's no social order and nothing to protect the weak from being preyed on by the strong. In fact, Mr. Cypher encourages this state of things, seeing the continual growth of stronger and stronger creatures to be the point of life. So, it's basically like Dominionist Christians v.s. the ultimate laissez-faire Libertarians. On the Neutral path, you can wipe both of these groups of idiots out and let humanity decide how it wants to arrange itself. At certain key points in the game you'll make decisions that put you on one path or another.
Anyway, you can see where no commercial entity will ever dare to translate this game and release it in an English-speaking market. Fortunately thanks to Aeon Genesis there is a translation patch (http://agtp.romhack.net/project.php?id=smt) available, but of course you'll have to get all piratey.
You play as this Japanese schoolkid, one of those "mute protagonist" deals where you're apparently supposed to project yourself into the role (though you have to be male in this one). You begin having this weird dream, in which you roll your own stats as well as those of a few allies (whom you will meet later). You then wake up in your room, where Stephen Hawking spams you with some Devil Summoning Software that allows you to store demons digitally and then summon them to fight by your side. A trip to the local shopping mall to pick up some coffee for your mom sets off the chain of events that leads to your involvement in this whole crazy war.
A lot of people who haven't played this game yet just look at the plot summary and go "OH MGOZORS THIS WILL BE TEH GREATEST RPG EVER", but you'll soon find the game engine is pretty clunky and requires a lot of patience. So much so that you might not find it worthwhile to claw through the game just to go one-on-one with the Archangel Gabriel or whatever. It resembles an American PC dungeon crawl much more than it does a typical Japanese console RPG.
The whole thing is first person perspective, except for when you're travelling between locations in Tokyo where you get a simple overhead map. It's got that whole "same walls and textures over and over again" quality that all these old dungeon crawlers use (think the first Phantasy Star if you haven't played any PC RPGs, or Shining In The Darkness) that makes navigation a headache at times, though a couple of hours into the game you get an auto-map that helps considerably.
The graphics are really simplistic - the backgrounds are low detail, and the characters and monsters you encounter are just one flat smushy sprite that usually has either one or no frames of animation. All in all, the game honestly has a pretty ugly appearance. The music is sort of generic ambient/shred rock, fairly well suited to the environs but nothing you'll crank the volume to hear or want to listen to later. The story, despite the bold setting, is actually pretty sparse and a lot of stuff just happens inexplicably or isn't adequately expanded on. The main character has absolutely no dialouge at all, and there's pretty much no emotional involvement whatsover. For example, fairly early in the game you come home to find out your mom has been eaten by a demon. After killing the demon the game is basically just like "Oh well, that's a bummer, back to business as usual. Let's go down to the mall and get a new sword". I mean, your mom getting eaten, you think that'd be a fairly major event but it has no more impact in the game than running into some goofball on the street.
The game is basically a hardcore level grinder, and you keep fighting the same monsters over and over and over. You're perpetually a little weaker than your enemies, so you have to make minions out of demons that you meet to augment your forces. Before each fight there's an option to try to talk to each demon. Each one has a certain alignment, and as long as they're not Pure Evil you've got a shot at recruiting them. You can take either a Friendly or Threatening tone with them, and the conversation goes from there. Each demon seems to have a particular sequence that you follow to convince them to join you, but it's frustratingly random. Doing all the right things might get you randomly attacked anyway at any point along the conversation tree, so you might end up abandoning the correct path and not trying it again. Also, successfully navigating the conversation branches isn't the end of it. Usually the monsters then demand all sorts of money and items to join you, and may still yet turn on you and attack after taking a couple of things. It's all very fickle, but one good thing is that once you have a monster under your control, talking to any of its brethren that you encounter will make them leave instantly and sometimes even hand over money or items to you.
Once you have multiple demons, and you've played an hour or two into the game, a location opens up where you can bring them to be "fused" together to create new demons. You can fuse 2 or even 3 demons together at a time, then can go on to pair the fusions with other demons to create even more bizarre critters. Usually these are stronger than the original demons, but it takes a whole lot of trial and error to figure out which are most useful. And now you see where Pokemon cribbed all its ideas from!
Truth be told, I gave this one about ten hours and then it was just too tedious for me to continue. It's a fantastic concept, but the actual game leaves a lot to be desired, unless you really enjoy repetitive grinding and oldschool 3D mazes.