Hi Troy,
So to reply to your first comment - you are correct, the majority of people using OS X wouldn't know what to do if they were required to use X11 to run your client. However, everyone I know (a bunch of engineers), would have no problem at all. So if you want to seed a version of that out to us, we'd be thrilled! (Remember that X11 comes with the OS)
On your next comment, despite your well-written explanation, the two of us who are reading this email still don't quite "get" the question. So, my response may not make sense, but I'll try. People who use OSX every day are used to things being the same from program to program (in terms of shortcut keystrokes, etc), so if you're saying that the look and feel would be slightly different than standard OSX, yes, this would be noticed. Would anyone care? Again, compared to not having the software, the answer is probably no. I doubt anyone could provide guidance on a toolkit that they prefer, since everyone is used to things looking and feeling the same and do not have awareness of widgets and toolkits. It's possible that the toolkits/widgets available are all refined to the point where there are no noticeable differences. Obviously, looking at something like Quicktime, it's developed by Apple so you know that would look right..
I hope that that helps, in SOME way.. :?