The only other difference I found is that the Passport Studio is pre-formatted as HFS+ while the other two are formatted as NTFS. This, and the Firewire 800 port is why WD labels the Studio as "for Mac". But you can, of course, use any of the drives with either OS X or Windows, you just might have to reformat it.
Edit: The Studio is also listed as Time Machine-compatible, unlike the Passport and Passport Essential, though I'm not sure if that involves anything else besides being formatted as HFS+ by default.
Edit2: as @Lèse majesté points out, the actual hard drives inside the enclosures could also be different. Note, however, that according to this KB article WD doesn't guarantee any specific hard drive model or any other parameter besides capacity. That said, from what I've read, most people who have opened their Passports up have found a Scorpio Blue inside (or a modified version with an on-board USB bridge). As the same KB article mentions, with USB 2.0 and Firewire the hard drive is not the bottleneck; however, with USB 3.0 that's no longer the case, so WD might decide to differentiate some external drive models from others by using faster Scorpio Black drives, for instance (if they haven't started doing that already).