My reason for wanting to switch was that Zotero has Google Docs integration. After making the switch, I was pleased to pay a modest amount for storage of my PDFs, which makes me feel like a customer instead of a product.
> 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.
If it helps I have just under 2000 papers in my Zotero. It isn't massive by research standards, but it is substantial.