All sides of this leave me feeling uncomfortable, to tell the truth.
On the one hand, Valve's announcement really just sounds like them saying they want to monetize mods on their platform but not be held accountable for anything that happens to go wrong. It sounded like they want to turn modders into contracted DLC creators, but without actually giving them any sort of contract. One thing that particularly rubbed me the wrong way was the comment that Valve wanted to do this so that modders could work on mods full-time. I'll admit that I don't have any sort of schooling in economics, but as far as I'm aware these modders would still be working out of their own pockets until their mods are released, at which point they would have to hope they could make their money back. From what I understand, that's not typically the way developers work.
Also, as anyone who has been paying attention to the storefront, Greenlight, or Early Access likely knows, curation isn't exactly a strong suit for Valve at this point in time, and while it might be unfair to preemptively attribute the same to a modification marketplace, I don't think it's outside the realm of reason to suggest that that would become a massive shitshow in short order as well.
On another hand, I don't blame modders for wanting to monetize their work. Personally, I think mods worth actually paying for are pretty few and far between, but I'm not really one to decide what people are willing to pay for. I don't like the idea of mods becoming locked behind a paywall, but that's more out of frugality and selfishness than anything else; I rarely pay for actual DLC released by the developers of a game, I find it hard to believe I'd be willing to part with my money for something that has a higher-than-average chance of breaking everything. I'm a bit worried about the potential for slippery slopes, as well, but that's mostly paranoia and speculation.
In a sprouted mutant third hand, I'm really annoyed by people who are responding to this by essentially saying, "good job, whiners, NOW MODDERS WILL NEVER GET ANY MONEY EVER I HOPE YOU'RE HAPPY!" Not only because they're ignoring the legitimate concerns some people had (which they occasionally even share), but also because it's needlessly antagonistic and just really unhelpful.
But of course, in the fourth hand that's dangling uselessly from the back of the head, there are a lot of people who really are just saying, "good, I'll never pay for mods ever, suck it!" And that just makes me feel disappointed. Not even surprised anymore, but still disappointed.
So, yeah. Uncomfortable, annoyed, and frustrated all around right here. Hopefully Valve will be able to come up with a better way of tackling this. I'd like exceptional modders to be sponsored directly by developers or something.