Etsy has a huge UI challenge right now: as the volume of listings increase, it becomes harder to find things that I'm interested in. Even text search is quickly overwhelming, because it's hard to describe new and quirky objects that are Etsy's biggest draw (for me).

If Etsy could create a good "we see you've bought quirky post-industrial greeting cards, maybe you'd be interested in ____" function (much like Netflix, Amazon, etc) then it would be greatly improved. It's a much harder problem to sovle than Netflix and Amazon because many items are one-of-a-kind (nobody else bought exactly what you bought), but comparisons of trends across sellers, across keywords, and across image features might work.

Etsy has an experimental "pick the object you like most" feature that leads to some results, but it doesn't allow narrow enough focusing yet: http://tastetest.etsy.com/

Same for the "explorer" view: http://www.etsy.com/explorer