One sign that you've done something with deep impact is if you spawn imitators. Star Wars certainly did that, as did Harry Potter. This usually creates a 2nd tier of successful ripoffs riding on the coat-tails. Galactica (classic) would classify as that for SW, and something like Percy Jackson would be the Harry Potter equivalent.
I'm not sure about this but did HP bring about the resurgence of fantasy and the like? Was the timing such that it could've affected PJ doing LOTR? I think it was around the same time.