A key feature for me! I'd like to see the ability to export&sync groups of papers, not only the complete library. Being able to customise the contents of exported data (e.g., abstract, custom note fieds) would also be nice.

Also agree. My manual system uses a flat file system with first author surname, year, and first (sensible) word of the title concatenated (which i also use for citation keys). I'd like to be able to replicate this in Mendeley.