Valve has come up with a plan to allow mod-makers to sell mods on Steam. By and large the community reaction has been unfavorable. Many gamers are unhappy that something which was free, is now going to cost them money. On the other hand, a lot of industry players feel that it's only right that mod creators are paid for their efforts.
My thoughts are mixed. In addition to theoretical changes, there are also practical issues with Valve's plan.
Practical Issue - Revenue Split
The first issue is the revenue split that Valve is using. Valve takes 75% and the mod creator gets 25%. Now, Valve is probably giving a portion of their take to the original game creators, which is reasonably fair. I've seen estimates that ultimately Valve gets 25%, the game creator gets 50% and the mod maker gets 25%.
I don't think this is a sound plan in the long run. I remember reading an article on startups (by Joel Spolsky, I believe) and he gave the advice that if you make a startup with partners, you should split the ownership 50-50 (or 50%+1, 50%-1 for control issues). At 50-50, both of you are truly partners, truly financial equals. Trying to apportion responsibility and unequal ownership makes individuals resentful. In the long run that creates more problems than being a little over-generous in ownership.
In the same way, I think that Valve should strongly consider a system where the game creator and mod maker are treated as equals, as partners. A split like Valve 20%, game creator 40%, and mod maker 40% would be better. You give up a small amount of revenue, but the mod maker is an equal, a full partner instead of a junior partner.
Practical Issue - Copyright
Most mods are not compiled. That means that it is really easy for other people to copy the mods. Piracy in itself will be a problem for mod makers, but people reselling other people's mods, or creating derivative works, on Steam will be a problem for Valve.
Hopefully Valve puts up a reasonable barrier of entry for mod makers. Something along the lines of Apples $100 dev licence would be a good start.
Theoretical Issue - Open Source
Right now, mods operate much like the Open Source software community does. It isn't explicit, especially with licensing, but it's very similar. The thing is that, by and large, the Open Source movement works. In fact, it's very arguable that in a lot of areas, open source software has swallowed up closed source software.
I think the mod community will split into two. One that is for-profit using Steam's marketplace, and one that is open source, using the current distributions. Mods may even be explicitly licensed with the GPL or similar.
I think the open source mod community will end up crowding out the for-profit community. They will have more users and be able to hit critical mass a lot easier than the for-profit mods will. As well, most mods, unless they rely heavily on artwork, can be duplicated fairly easily. Think of how many different DPS parsers exist. If a for-profit mod becomes popular, I imagine that a free mod with the same functionality will appear quite quickly.
Perhaps ultimately most mods will be free and open-source, but a Premium version with a better user interface or extra options will be sold on Steam. Very similar to how many current open source programs are sold.
There is nothing wrong with selling mods on Steam for money. In fact, it's probably a good thing that mod makers get rewarded for their time and effort.
However, in the long run I don't think it will make much difference. The pressures that push towards open-sourcing software will exist in the mod community. Most mods are written not for profit, but "to scratch an itch" for the modder. That same philosophy will still exist.
This does not really apply to artwork-heavy projects, or massive mods which essentially change the game into something new and require a lot of content creation time. This might very well be a good platform to sell those types of mods. But realistically, there aren't that many of those mods made.
Of course, now that money is involved, anything that requires a team is going cause massive organizational headaches for amateurs. You may still see massive mods being released for free, just because everyone who worked on it getting nothing and doing it for fun is easier than trying to pay individuals for their contributions.