I'd pay $10 a month if addition to pasting a doi -> pdf, that I could also search backward and forward along citations. I'd love to be able to download a paper and everything it references.
Iirc that's pretty much why TBL invented the web, so you could click on references in scientific papers. It'd be nice if we could actually do that.
I'd probably be willing to pay $30/month for that. Of course they would have to actually have all or most of those backward and forward references available.