Lazy loading?
For larger libraries, it would be really nice to have a "lazy loading" feature to be able to use zotero on smaller hard drives (e.g., netbook). Instead of downloading everything on a sync, zotero should have an option to selectively download attachments (the whole index is fine, of course) and delete them without removing them on the main file server.
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Upgrade Storage
One issue that makes selective downloading important is that large group libraries are an untenable proposition so long as group membership (with file sync) requires keeping a local copy of the entire set of attachments, and so is certainly on the long-term agenda.