The favorites thing is definitely going to happen.
I’m working on a bunch of updates for the site right now, and adding favorites is on that list. My main work is updating the site to the newer version of Django so that I can keep making updates to the main codebase. I wrote about it here a little: Feedback, suggestions, say hi! - #228 by peter
I realize a 5 star rating system isn’t perfect, but I also don’t think just a binary system is right either. I’ve always liked a simple 5 star rating for myself, so that’s why Grouvee has one 
I’ve also made it optional to use the ratings, so hopefully having a favorites button will be a decent compromise for what you want to use Grouvee for. I think our shelf interface is the easiest one to use out there, so hopefully as we tweak things here and there, it’ll be everything you want it to be.