You people have different phones?
Mine came with 1.something Gb in "other" from the box. I just left it there, it now floats around 1.5G. I'm assuming it's caches, temps, and common data.
I also remembered that my "app" data never grew when adding accounts, so I'm guessing things like OS apps store there - don't know about you folks but I have a TON of emails. I assume some of that stuff (mail, contacts, local backup before being uploaded, temporary data, drafts, etc) all end up there. Xbox data, like caches of avatar customization? Unpacked versions of jpegs for use as lock screen? Cached music thumbnails? Cached artist lockscreens?
I only have about 100M of map data, but conversations are fairly large, as I have images in them. Contacts also feature a few hundred images, mail is packed, and so on. No idea if that goes under OS, app or "Other"