I have started the development of a greasemonkey script that tries to enhance 500px, which is already wonderful.
It's name is Better500px
and can be found here: http://userscripts.org/scripts/show/99057
First feature already included: on a photo page, the heart for favorites is moved to the right, closer to the "like" button.
Second feature I'm currently working on: showing on list pages if a photo is already a favorite or not. Would be much better with an API, a bit too ugly right now for a public release.
I will add features from time to time...