I wholeheartedly condemn this move by Valve. It is nothing but a cash grab attempt from Valve and Bethesda, in the garb of "helping the modders". If at all they wanted to help, simply adding a donation option would have sufficed.
Making mods payable essentially divides the player base into those who are rich (and hence have access to paid premium mods) and those who can't afford to pay / not willing to pay and would have to settle on "free" mods, which in all probability would not match the quality of the paid ones (just see any App store). Down the line, more than 90% of the modders would expect players to pay some basic amount for their mods. Who can resist the free money?? Another point is, since most modders develop mods as their hobby / interest, and hence for creative satisfaction, why try to monetize that?
Modding has always been one of the strongest points in favor of PC gaming, compared to consoles. If Valve is planning to add more games for paid mod option, then this changes the entire scenario for PC gaming, and not for good. I am hoping the overwhelming negative reaction from the PC community will force Valve to reconsider their decision, or at least alter it sensibly.