If it weren't for the non-action bit, I might have suggested Fallout New Vegas, as it's a massive, massive RPG with quite a few options and fairly recent. FPS, but VATS makes encounters much easier.
Fallout 1 and 2 are exploration based as well, and are turn-based, but also older.
Not exactly elaborate skill systems, but sometimes less is more. Otherwise I draw a complete blank for massively open, explorable RPGs. Kinda sad. Until I looked at this thread, I hadn't realized that I can't think of any exploration based games besides ones from Spiderweb and the Fallout series.