The best modern RPGs you'll find on Steam are Knights of the Old Republic and Morrowind. There are some older RPGs too, like Fallout, that are very good. I usually go to gog for my RPG fix so I'm probably leaving some out.
These aren't jrpgs, but judging by your taste (questing, story-driven RPGs) they might be right up your alley. Legend of Grimrock is amazing but it's strictly an old school dungeon crawler.
Check gog.com for a better selection IMO. No download necessary other than the games, and they're drm-free.
Edit: I am an idiot for initially leaving this out. Do yourself a favor and buy Phantasy Star 2 and 4 right now. They're both on Steam for cheap, they're old-school jrpgs, and IMO they outclass most Final Fantasy games. You won't be disappointed.