Gareth, I would say that the biggest challenge facing any OGL publisher is that the breadth of the customer's tastes does not approach the breadth of the market.
I think it would be a rare gamer who was equally interested in Forgotten Realms-style high fantasy, Eberron-style contemporary fantasy, Greyhawk-style swords-and-sorcery, Black Company-style low fantasy, historical gaming, pulp gaming, space opera, swashbuckling adventures, hard science fiction, post-apocalyptic gaming, et cetera - plus all the variations I haven't named, plus the different OGL-based games serving those variations.
As a subscriber to Dragon, I can say there were always multiple articles in each issue which weren't interesting to me in the slightest - and that's just in a magazine devoted to revised Third Edition D&D as published by Wizards of the Coast!
Games based on the OGL are often significantly different from each other. A feat in Dungeons & Dragons isn't equal to a feat in True 20 isn't equal to a feat in Conan, and there are no feats in other games like Castles & Crusades. Any magazine that serviced the OGL market would have to make decisions about which kinds of OGL content to include, and every such decision cuts down on your audience.
This applies even on an issue-to-issue basis - it's only because Dragon was so consistent about providing articles I was interested in, and because I knew I was interested in the Savage Tide Adventure Path from Dungeon, that I subscribed at all. Otherwise, I would have been another gamer who only bought it off the shelf when it had enough articles to interest me - and an OGL-focused magazine can't guarantee me that I'll be interested in something from every issue, or even (I suspect) a majority of them.
It's not just about the rules, it's also about the content. You could write a D&D-compatible article about drow, and I wouldn't buy it because my D&D game won't be using drow. Then you could write an article about vampires in Conan, and I wouldn't buy it because I don't play Conan. Dual-, triple-, or quadruple-statted articles won't work either. They barely work in situations where there's an established fanbase for both versions of the game, like AEG's Legend of the Five Rings products feeding on the original game and Oriental Adventures.
I think Paizo Publishing has the right game plan here - with Pathfinder, they're focusing on providing a "core swords & sorcery" experience, a traditional-D&D-focused publication. This is by far the biggest and most important segment of the market, of course, but by publishing under the OGL they really are able to take anything they like from the wealth of OGL material out there and change, improve, and re-purpose it for their own aesthetic vision of the game.
I just don't know if it would work for any OGL game other than D&D, and if it can work for a magazine as opposed to a tightly-focused Adventure Path-based publication (or something similar).
Maybe I'm not making much sense.
