It's not a technical limitation, it's a culture limitation. Mac apps care far more about providing a consistent UI than Windows and Linux apps (although I believe those platforms have improved in this regard in recent years). It's certainly possible for a Mac app to muck about with the non-"app" parts of the window, it's just far less common.
I meant it tongue-in-cheek, but what you say is true and the Steam interface has always been a custom one. I haven't used the Mac version - how did they rectify this on the Mac?
The Steam UI on the Mac looks exactly like it does on Windows. They skinned the whole thing. And this isn't a bad thing. Steam is intended to be a single cohesive gaming platform. It should look the same on every OS that it runs on.
I really don't think that is true at all.
Agreed. It probably had more to do with the fact that their store, game details, etc. views were pretty complex. They seem to have solved a lot of that crossplatform pain by implementing most of that stuff using Webkit.