I don't know if this has been described or discussed (I've been trying to keep up with this topic and the AMA, etc.), but how will updates work? If you buy a mod once will you be entitled to all future updates to the mod, even significant ones?
A lot of mods (especially bug-fix ones) are updated regularly whenever there's a new patch, for example, and new issues pop up in the game or new features become available. So if you have a mod creator who comes up with "Skyrim Bug Patch 1.0", will everyone who buys it be entitled to version 1.2, 1.6, 2.1, 3.0, etc.? And if so, aren't there diminishing returns for the modder after the initial release, because s/he isn't receiving that much more revenue for the extra work s/he's putting in? (I guess the same way there's diminishing returns to the developer / publisher for bug fixes, which is why you don't see patching being a huge focus post-launch in the first place...)
I can see "responsible" modders having to put in a lot of extra work for very little money to satisfy their "customers", and other modders just abandoning updates because it's not worth the extra time and effort. With free community mods I feel people are willing to pick up the slack or pitch in when a modder gets overwhelmed, but obviously in this case that's complicated if the original modder received the lion's share of initial revenue.
Also, it seems likely that people will feel "locked in" to the first version of a mod they purchase - because there's no way I'm going to spend $5 on "Bug Patch 1.0" and then another $5 if someone comes out with "Improved Bug Fixes 1.0" the next day. So I guess it might lead to a "gold rush" mentality of being the very first person to come out with a mod (which might be good for speeding up mod creation), but it also dis-incentivizes anyone who wasn't "first" from contributing further and potentially better improvements, because purchasers will feel locked into the mod they started with (which I'd consider a negative).
I'm rambling a bit, but it really is uncharted territory here and I'm not wild about some of the implications of moving towards a paid model where mods begin to resemble, for example, the mobile freemium market. Nor am I reassured by a philosophy of "the market will take care of it, people" - it's not a true "free market"; it's a curated market overseen by Valve, the same way the App Store is a curated market, and Apple's submission and content policies and search algorithms have dramatic effects on what gets seen and downloaded and what financial incentives get created.