Hi!
I grew up in the Seattle suburbs before starting my MS in Engineering at Illinois, and I totally understand some of your concerns.
Mountains: Yeah, there are no mountains, and I miss them. But cornfields are their own type of charming; they've been growing on me, but will never replace the majesty of mountains.
Bars: Did you make it out to downtown Champaign? I know the Green Street/campus bars can have fratty vibes, but there's a lot of diversity beyond those!
Facilities: I was surprised by this one; I too went to a smaller, private, religious school and I am constantly amazed at the plethora of resources at my fingertips now!
You mention having grown up in a liberal, outdoorsy town, and that culture IS present at UofI! It tends to be on the Urbana side and may be more present in some departments than others. Despite its shortcomings, I've grown to like the area and I think there's a nice balance between Champaign-Urbana being interesting enough that I don't get bored, but not so exciting that it distracts from my work.