This really is a tough one, we were all there once, but anyone hanging out on the forums were the type to get past the struggle so it's a bit hard to judge. I'd say maybe highly emphasize the bay12 forums, maybe even a popup at startup (able to disable it of course) similar to how Streets of Rogue does. Basically you want newbs to know that it's ok to not understand everything and we're willing to help out. Somehow without stepping on toes, make it known that we aren't the cesspool that steam is. Make it easy to report "bugs" even though I'd wager most things aren't actually bugs, just not understanding things. You don't have to understand everything to start playing. You learn as you go. Gaming is basically so oversaturated with choices that it makes people very unlikely to give a toughish game a fair chance.
The one thing I could say, maybe a tutorial, no idea how feasible, but maybe a tutorial scenario with no enemies just to get people used to beginner stuff like farming and learning designate, etc. Some way to include stuff like how stairs work etc. And always emphasize in the tutorial that we're happy to help on the bay12 forums. A lot of this stuff exists, so maybe talk to some of the folks who have made good beginner tutorials and see if they would be willing to make a tutorial map pack or something.
I'd say whatever you do, don't get discouraged because while steam is almost a necessity to grow and reach a larger audience, they have a very fickle culture that's full of trolls. Depending on finances, it might even be worthwhile to hire a few people to respond to bug reports or other things.