Lots of text on how Steam rocks and GOG sucks, but you still failed to answer the original question.
It' the first sentence: "I was initially drawn here by Fallout 2, Baldur's Gate, Jagged Alliance and Heroes of Might and Magic III (which were hard to find) when GOG was the only place that had them
" That's why and how I got here.
About a year or two ago, GOG also had two sales in succession that had discounts that were better than Steam (for the same games.) I actually bought a bunch of games from GOG from those two sales. I was hoping it would become a trend. Unfortunately, it did not last. It only happened twice, and has not been repeated since then.
Anyway, my points:
(1) For the same games that are available on GOG and Steam, GOG should have the same levels of discounts as Steam/Green Man Gaming/GamersGate does. (Which has not been the case except in two sales; as a general rule, the discounted prices of GOG are higher than those on Steam.)
(2) If GOG gets an on-going series long after Steam has it, GOG should offer "incentives" for people (who can prove that they already own the series on Steam) to migrate.