This feels like a concept that is fine, in theory. Modders should get paid for their hard work, and nobody should feel forced to give away something for free simply because "it's the standard".
However, there are several issues with this particular implementation, and several more with the way some modders will inevitably implement it themselves. First off, and this is something I covered in the other Skyrim-related thread, is the fact that the decentralized nature of Valve's structure means that nearly all of these new features have the nasty caveat of "...and then users will just regulate everything themselves", most probably due to the fact that these projects are done by small teams who have the ability to place these features on Steam, but not the ability add new employees to the company's payroll to oversee regulation of these new systems.
The issues with legality are all but inevitable. The modding scene is extremely collaborative, even in terms of "passive" collaboration where people freely take open-source mods and integrate them into their own open-source mods as they create their own larger vision or iteration of the same basic concept. It's this free-to-access nature that lets modding flow from one person, one iteration, and one concept to the next. Any person at any time can pick up the torch and provide a new spin or advancement to the same project.
That's not to say people who want to be compensated for their work are bad, greedy, or "against free modding". Those modders are well within their rights to charge or do what they wish with their mods. But the real issue begins with a paid mod that depends on the work of a free mod. It's easy to see how this wouldn't be a problem if someone's paid mod was built from scratch, but what happens when someone who wants to be compensated for their mod work is using some portion of a different mod that was explicitly open source? Would the author of the original work really be OK with that? Because they don't really desire to make money off their work, does that mean they wouldn't or shouldn't be bothered when someone takes some part of their work, modifies it further, then sells it?
Those aren't questions that are answered easily, and those aren't questions that Valve has even bothered to ask, judging by the fact that they hoisted something like this on the Workshop without actually having figured out half of these kinks.
Exactly. Their "pay what you want" option isn't really "Pay what you want", it's "pay something and we'll decide how much goes to who". Part of Humble Bundle's appeal was letting people choose to whom their money went to. All to developers? All to HB? All to the charity? Any combination of the above? Go for it, you're the one paying, you have the choice.
Valve's doesn't work that way. I want to give $5 to X modder for his really awesome work except I can't. Only $1.25 will actually go to to the modder, the rest is Valve's/Bethesda's. Literally the only way I can give more money to the modder is by HAVING to give Valve/Bethesda even more money in the first place. It's really sleazy.
I honestly wouldn't be surprised if modders who pick the "pay what you want" option eventually just make their mod free, then include a donation link in the mod's description. They at least would get far more than a measly 25% for any and all donations.