What makes a bad reference manager?

Update 2013-11-11: For some statistical data read On the popularity of reference managers, and their rise and fall Update 2014-01-15: For a detailed review of Docear and other tools, read Comprehensive Comparison of Reference Managers: Mendeley vs. Zotero vs. Docear

At time of writing these lines, there are 31 reference management tools listed on Wikipedia […]