1) If the people who made the mods want it to be free, it should still be free. If people want to charge for their mods, they should be able to charge for their mods.
2) Steam taking 75% of the payment is reprehensible for a game they didn't make, for mods they didn't make. Basically, they're taking 75% for bandwidth costs. Absurd.
3) If you have a problem with how Steam does this, I strongly suggest everyone buy physical, and simply link it to steam, as opposed to buying digital and allowing steam to control the content. In this regard, you can still go to the vastly superior mod sites, and still get the benefits of steam where/whenever necessary.
4) My biggest concern over all of this is that slowly the modders who do it for the pure passion, will be slowly forced out. If the steam setup becomes super popular, you could see devs start adding mod tools back into games...but at a price. Though, I think it's fairly safe to say that many modders are content with working on the game they currently are modding for...so, it could simply encourage them not to take interest in newer titles.
5) For me, it smells like the beginning of how they took server control away from users, and started charging for it. That's why I have a very serious concern about this. But, as long as I can still purchase a physical copy and cut steam out of the equation, I have no problems.