Meanwhile, in the hours since Chang'e 3's successful landing, many people have worked to figure out, from the descent imagery that was shared live with the world, precisely where the spacecraft landed. It's a Sunday morning so rather than summarize all of this great work I'll simply link to some of the best.
Geologically speaking, the lander came down in Mare Imbrium, at an interesting spot: very close to the boundary between high-titanium and low-titanium basaltic lava flows. The rover should be able to explore the nature of this boundary. According to an email from Moon mineralogist Carle Pieters, they are sitting on the younger, high-titanium basalt.