The differences between the two are so limited that it could be said that the two genres are too similar to differentiate. In this generation of multiplicity the two forms are being mixed, thus the idea of an RPG puzzle game is common.
Or action-RPG. Or RTS with RPG elements. But yes, I agree -- adventure and RPG are so very close; it seems the deciding factors that differentiate them are the superficial ones, like leveling, loot-gathering and character customization (stats). Here's my definition: if you have wriggle room to develop your characters, stats and personality wise, and your choices can affect the story/quests, then it's an RPG. So for me, Diablo 2, Dungeon Siege, and the Final Fantasy games are not RPGs.
