PoSSeSSeDCoW: I made maps for the original Starcraft - it really wasn't hard and it was quite fun. There was a cornucopia of maps for SC, monetizing it will merely reduce the map pool (no one will actually buy the maps).
The success of DLC on consoles and simulator add-ons on PC says otherwise. People, especially gamers, tend to be spoiled and want everything they can get their hands on. That's why crap sells that shouldn't far too often.
If modders make awesome maps that take a lot of time and are obviously better than free maps, people will pay for them, and really why not? I'm not one of the PC gamers who demand free content, I don't mind paying for things that are worth paying for.
As to someone else's point about paid maps throwing off the community and such, I doubt that will be the case. The best maps will be bought by the majority of hardcore players, and random casual maps will all be free ones, I suspect. Plus, it's kind of silly to not think Blizz has a plan for these things... everything they do is well thought-out and made with quality, remember.