Gamasutra - Clone vs. Genre
This article at Gamasutra looks at the line between games that are simply clones of a seminal classic - or part of a valid genre. While RPGs aren't mentioned at all, I thought the place of "Diablo clones" might be worth discussing:
We often talk about “clones” of games, or copies of ideas -- but at a certain point, if an idea is copied, expanded, and massaged enough times, nobody says "clone" anymore.
When does an idea become large enough to lose its identity and flourish into a full-blown genre? Perhaps it’s better said that the idea gains a new identity, one that’s more generally applicable.
Most recently, we stopped referring to "Tower Defense clones," and began discussing tower defense games as an entire genre, which has since been expanded to include a number of variations by companies as diverse as Square Enix and PopCap.
"Doom clones" stopped appearing around the time Quake came out, and we started to deal with the first-person shooter as a genre.