I know of three open systems other than d20. They are the Action! System, the Iridium System, and FUDGE. There may be a 4th, but I didn't catch the name.
The Action! System is not well known. Nor is the Iridium system, and it has the further drawback of being a bit too busy for its own good. Fudge has the problem of being the Linux of RPG systems. A good tool kit, but to do anything with it you need to put the kit together.
Haven't seen the upcoming Runequest system so I can't comment on that.
In order for a non-d20 system to catch on it would first have to be connected with a product that attracts an audience. And I have yet to see any RPG that does that well. Even D&D draws more thanks to reputation than presentation.
The system itself would have to be coherently done. In other words, organized. I have HARP Lite. HARP Lite is not organized. You have to jump from page to page just to put a character together. Rules are scattered hither and yon. From what I've read so far of the Iridium System RD, for all I disagree with many elements of the mechanics, it is much better organized (and better written).
The game itself would need to use the system to best advantage, with only the minimum changes necessary for setting and genre. Best would be the addition of mechanics designed to model any special conditions existing in the game world. Worst would be altering the core mechanics because they don't suit the milieu.
So for a non-d20 OGC game to succeed it has to do the following:
1. Draw potential customers who have no prior knowledge of the game
2. Present a system that is clean, organized (can't stress that enough), and is comfortable to use.
3. Offer a compelling game world.
Now where the system and the game are concerned each must answer some questions. Two in the case of the system, three in the case of the game.
The system must answer these two questions:
1. What do you do?
2. How do you do it.
The game must answer this question:
1. Where do you do it?
Any RPG that can answer that question, any system that can answer those questions, in a comprehensible manner has a huge headstart on systems and RPGs that cannot. Add in a presentation that says to the prospective customer, "Here's something you may find interesting." and you could have something that can actually fare as well as D&D.
As an exercise in critical thinking take your favorite RPG and answer the question, "What do you do?" I, in turn, will tell you why you're wrong.
![Devious :] :]](http://www.enworld.org/forum/images/smilies/devious.png)