How does ReadCube compare to Mendeley?
From a short comparison of the features listed on the website here is list of Mendeley features that ReadCube doesnt have:


In-text citation insertion and formation: Mendeley has Word and other plugins, ReadCube only exports to Endnote and "your favorite citation software"
Sharing and collaboration features
ReadCube is only a desktop app (no web app), and it doesn't work on Linux
Mendeley has an open API to query its huge database


On the other hand:


ReadCube is used as the online reader for a few publishers: Nature, Frontiers and Wiley.
ReadCube suggests personalized article recommendations (dont know if they're good, though)