Sorry this took so long, real life has been insane for the last couple weeks.
The main point I want to cover here is this: Best d20/OGL and Best Game don't cover the same things. It's not that there's d20 and then everything else, it's that, by their very nature, d20 games don't qualify for Best Game.
In order to qualify for Best Game the product must be a complete gaming system. Though I'm not really sure if that has been the case. The Best Game award hasn't always been a part of the ENnies. The awards began as a d20 award and it shows. If you look back as far as 2004 it was Best d20 Game and Best non-d20 Game.
Obviously there was a problem with that, and it's changed since then, but it was before my time so I can't speak to it with any authority. As long as I've been part of the awards the rule has been in place. Because of this d20 games don't qualify for best game. By their nature they are not a complete game system. Now, some OGL games do, but even those were pretty rare until the last couple years.
The two categories are closely related. Both cover all the little things that make a game book a complete package. For the most part this includes rules, writing, and presentation. They do however have a slightly different focus.
Since Best Game requires a complete system it requires a certain level of comprehensiveness. The system must cover all the reasonably necessary elements of game design and play. I'd also argue that this category could cover more than one book. For example, games like Dungeons and Dragons, Burning Wheel, or GURPS have multiple core books. As individual entities books like the D&D Player's Handbook don't qualify, as they don't contain a complete game system, but as a set of core books they do.
Best d20 however doesn't require a complete game system, so it allows for a wider variety of products. This lack of focus is probably a weakness in the award itself, but it can be helpful. When I was a judge in '07 at least 2/3 of the products we received were d20.
Not that it matters much at this point, since the Best d20/OGL award has been removed from the ballot this year, but I thought it was a discussion that needed to be had. I also think there is room for an award like this, if not this award itself. There are a lot of core books out there that don't include a complete game system, things like the World of Darkness games or Savage Worlds settings as much as the D&D books. I wonder if it wouln't be a good idea to include an award for those sorts of books? One with more focus than a d20 award, and with less singling out of a specific product line?