The big draw for me with the Kindle that doesn't seem to be available on its competitors is wireless document delivery - I used this with Instapaper, and now with Readability. Thanks to it, I never need to worry about plugging my Kindle into my PC to add articles I want to read. There's also some interesting content in the curated "Kindle Singles" section, cheap short stories, novellas, etc.
Honestly, I switched because I liked the interface and the mobile web app more than I liked Instapaper's site and app. Readability's "readable view" is soooo much nicer than Instapaper's. Supporting authors is a nice bonus, though.
