honestly $20 is going to seem too expensive to the steam community. people like us who already have this game know it's worth waaay more than that, but the average person looking at the store will think "meh, there are better games I could get for that much"
seriously doubt that this will make the community skyrocket like a lot of you say it might, but either way it's about goddamn time it happened.
there will likely be a burst of sales at the start, people will yell "omg look at all the new people" but it will die down pretty quickly. this happens with pretty much every game ever. expect bursts of new users when the game goes on sale, too.