You're right about Pratchett. I'd forgotten about him. I've never read any Lovecraft. I admit that I'm not an avid fantasy reader, so I don't really know what I'm talking about.

I've got a few discworld books, but I could never get in to them for some reason. I got a beautifully illustrated hardback Pratchett book for Christmas one year. It was nice to look at, but I couldn't get in to the characters...
You kind of have to start at the beginning of their book series to get into them, they have a lot of progression.
H.P isn't original, he based lots of lore off of scienctific theories. Or so it seems to me. Nothing is original in this world, nothing. It's all a matter of how close to being a copy something is. It's a weird thing about the universe, or at least the human way of vieing the universe.
http://tvtropes.org/pmwiki/pmwiki.php/Main/TVTropesWillRuinYourLife I can't find the right page on TV tropes, but it expains the concept that nothing is original well. This is the closest I could get.
That's sort of a depressing view.
I suppose at least 50% originality is enough to enjoy without constantly thinking of the original concept.
The problem with originality, or if you would, pioneering something, is that sooner or later someone will catch up with you. Rip off your works, maybe even pull them off a little better and so on. So even if you did something "first", and that fact is acknowledged, it doesn't make you original when you're caught up with.
There is a difference between being original and being unique.