I'm curious, what legal issues come to mind? We've thought through many and what we always land on is "it's a gray area". Very little precedent and lots of existing services with similar functionality.

Copyright primarily. Fair dealing/usage doesn't protect commercial use except in fairly limit circumstances. Generally search engines get away with it because of "implicit licensing" and the fact that content owners want to have their content indexed.

When you're using the content of someone who doesn't want to be indexed, there's no real legal defence for creating a derivative work.

(IANAL but I took a couple of courses in IP law at uni)