"Other" includes any/all of the following: email account data, documents (be they attachments or storage cached from SkyDrive and SharePoint), browser history/cache, downloaded maps data, application settings/data (for example if an app downloads a lot of cached images for live tiles or lockscreen art, etc.), and pretty much all the non-categorized stuff. It sucks we can't "see" more specifics, and that's what I'd like to see improved, but my guess is you sync a lot of email data and attachments and/or maps and browser information. You could try the following:
- Settings > Applications > Internet Explorer > Delete History
- Settings > Applications > Maps > Delete History
- Settings > Applications > Search > Delete History
- Settings > Applications > Reset Office
I read a couple of the responses above and see things like "I only had 450MB free" (not picking on one person, just using an example) and think to myself, "WTF?! Remove some content, install update, and sync the content back again. YOU chose to install so many apps and content." Some of you are just complaining to complain because you want a piece of plastic and silicon to be smarter than you. "Gee, why can't I use my phone without consuming disk space?! WHAT?! YOU MEAN APPS TAKE UP SPACE?!" Good grief people, think!!