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severe downside, including but not limited to issues like opening you up to discovery
If I was opposing counsel, I know what my RFPs and rogs would include. And there's always the possibility of depositions.
To be fair, there's a ton of copyright cases that basically say that game rules aren't copyrightable, and some of them might be moderately on-point. The argument would then be that the OGL is a trick -- it's offering you special terms under which you're permitted to do something you could have done anyway, etcetera.
I wouldn't want to be the one to argue the case, in general. But there is definitely previous legal history for the notion that the rules of a game can't be protected by copyright, which is why so much attention goes into the brand identity stuff, which is much easier to defend.
Disclaimer: I'm not a lawyer.
Like I said here, text does not become public domain simply because it incorporates game rules. It's the procedures & processes described by the text that are not copyright protectable. You can't just copy the text of someone else's game rules, that will infringe their copyright. This is so even though in theory you might be able to create a game with identical rules to theirs without infringing their (c).
To be fair, there's a ton of copyright cases that basically say that game rules aren't copyrightable
That is roughly how I assumed it would work, but the rulling in Allen v. Academic Games went significantly further than that. It's not exactly precedent in most circuits, and I don't know whether it would hold up today, but they did pretty directly conclude that you can in fact simply reproduce some of the unaltered text of a game rule in order to describe the game.
And of course, when people talk about making D&D modules, they aren't proposing to duplicate the text of the D&D rulebooks; they're talking about using words like "strength", or saying things like "this character is a level 4 fighter", and it's really hard for me to see how that could possibly infringe a copyright on the core books...
In fiction, what this means is that the plot of your work is not covered by copyright. If Hamlet were written in the 20th Century, the musical West Side Story would not be violating copyright.