I like the Whitebox rules for Swords & Wizardry. They are simple and to the point, streamlined and yet they give you plenty of options for playing a fantasy game. My only complaint is that there isn't a thief class, which is understandable since the original game that it is emulating didn't have one either, but it is easy enough to fix this.
I ran a Whitebox game for a while on G+ Hangouts, until we "upgraded" to the Core version of Swords & Wizardry. I have a lot of respect for what can be done with a very few rules in a game. I went through my period, like a lot of gamers I think, during the 90s when I wanted a lot of rules in my games, even when I didn't really use them. The rules were comforting in a way, letting us all know that thing were covered if we needed them to be. Nowadays, that style just isn't my thing. I've played this way for a while now, but I have definitely become much more comfortable with a "rulings, not rules" style of play over the last few years. There's a trust there that everyone at the table is there to have fun and not screw each other around that I kind of like again.
So, Atlantis!
Most of my Swords & Wizardry gaming has grown out of my enjoyment of a sort of pulp style of fantasy fiction: Moorcock, Carter, Leiber, and Howard forming a lot of my ideas for what fits into a fun fantasy role-playing game. This makes sense since it is the sorts of fantasy that the designers of D&D were reading as they played and created the game. It is a fun style of fantasy, heavy on action and magic (some of the times), and people living their lives large. Beautiful women used their charms to bewitch the heroic leads, almost always against the backdrop of a world of ancient, decaying graces that are giving away to a new age.
That is what I am looking at with this game. Atlantis is sort of like my analogue to Moorcock's Melnibone. Except where Melniboneans were cruel and inhuman monsters subjugating the world through terror and magic, the Atlanteans brought culture, society and order to the world. This sets the stage for the Law versus Chaos conflict in old school fantasy games, with a world where the premier civilization is in its twilight days, but the light has not yet gone out. A world where chaos and weird monsters are again peaking out from the shadowy corners of the world, those things that will take stalwart adventurers to hold back. The great cycle of the conflict between Law and Chaos turns, and the world is changing....but into what? That, of course is up to those bands of adventurers that are around the world, trying to hold back the night.
From the Far East, there are the blue-skinned Strangers, with their sky ship and their strange weapons, and stranger magics. Some have even seen that there are Strangers with more than one set of arms, even. There are disenfranchised Amazons, wandering the world and seeking to right the wrongs within this darkening world, and bring their strange Amazonian ideals of peace and love through strength to the rest of the world. Barbarians are everywhere, causing destruction and seeking the chaos that can be found in the world. There are still the Atlantean Sword Brothers and Sisters, some of the best warriors to be found in the world, selling or giving the swords and their bodies to keep the light of civilization from winking out completely. There is magic. There are monsters.
One thing that I find about an old school fantasy approach, particularly with older versions of D&D, is that they tend to be happening during the apocalyptic times, when things are going bad but there is still a chance for change. That is one of the things that influences Atlantis! in my mind. This is not some idyllic time. This is not an age after things have happened and people are trying to survive. This is the time of change, the time of battle, when strength of mind, of body, of will can make a difference to those who are willing to try to make a change in the world. And that, to me, is one of the things that makes RPGs so much fun. You can be that person who steps up and says "None shall pass!" and make that stand, whether or not the odds are in your favor. Hopefully, this will come through in the final version of Atlantis!