I think the new 'My Ebay' format is to partly blame for this lethargy when it comes to leaving feedback. You now have to actively go to your sold and purchased sections, rather than before when the last few items were always visible as part of your 'My Ebay' page.
Ebay is now just yet another online retailer and you don't leave feedback for items you buy elsewhere online.
"I should say I`m very much cleverer than any of the people who put me here. As a matter of fact, I could leave any time I wanted. It`s only a doll`s house after all. Anyway, I don`t mind. I like dolls" - The Mad Hatter, Arkham Asylum.