What do you mean "domain specific"? Personal subscription is also available. And of the popular online reference managers, I don't think open source by itself certify anything related to the core functions of a reference manager. In my opinion, the really established open source ref manager is BibTeX, however, its user base is quite limited even inside academy to specific fields.
> It includes a Word plug-in, from which you're able to directly search PubMed [...]
That sounds like it's at least focusing on medicine as a field (and generally fields in which Word documents are an acceptable means of dissemination...)
Regarding open source:
Being open source is certainly not core to the functionality of a reference manager. But it does protect you from exactly the kind of behaviour shown by Mendeley here. So recommending a "solution" for the situation that has exactly the same drawback as the Mendeley (i.e. vendor lock-in) is short-sighted in the extreme.
