Read the X-Men Marvel pocketbooks in the leadup to Christmas. The Chris Claremont/Dave Cockrum/John Byrne stuff.
Looking at the plot twists, emotional angst, relationship entanglements and groundbreaking storylines such as the Dark Phoenix epic and Days of Future Past it became really clear that Joss Whedon really learned a hell of a lot from Chris Claremont.
And he put it to good use in Buffy. Not quite convinced by his work on Astonishing X-Men but he might still win me over.