Besides the obvious: “It’s not open source!”, I’m also not making the switch from Zotero to Mendeley for my academic reference management needs due to the answer to this question on the Mendeley FAQ page:
Is Mendeley free?
The straight answer would be yes and no. Yes, it’s free, because: Everything you get when you sign up to Mendeley is completely free and will always remain free - including the features described in What is Mendeley?
No, it’s not completely free, because: At a later point in time, we will expand upon the existing features and introduce additional ones for professional users —- these will be available for a (very reasonable) fee.
It’s also not free as in freedom…
