To be honest, I think what the op was asking is a way to extend the Mendeley Desktop client, similarly to what Firefox add-ons do, on top of being connected into the Mendeley API (which is a GREAT thing to have already *thumbsup*!). So, would love to have that at some point :)

This is indeed unfortunately not possible at the moment.. This is why we've created an app based on the Mendeley API to allow reviews/ratings of articles - http://www.papercritic.com - maybe one day this will be integrated inside Mendeley :)

Actually - this is already possible by doing "Add documents" - and then just selecting a file containing BibTex data.. obviously it would be nice to be able to just paste a bibtex entry when importing "manually".. but in theory this is pretty much supported.

I can't understand how this is not supported yet - so many people/sites have publication data in bibtex format and importing those documents into Mendeley would be so infinitely much easier if this notation were supported. Hopefully this comes soon!