As in OS X users are less tolerant of badly designed GUIs?

That might be a factor, but no, mainly as in "tk interfaces look and behave dreadfully on OSX, as do pretty much all X applications but even more so, when they accept to work at all".

* Even from on a hot start, gitk takes almost 5s between invocation and startup (on a 2.4GHz MBP) where GitX takes roughly 2s on a cold start (cold-start gitk takes on the order of 10s).

* X applications don't behave in OSX: Divvy's resizing doesn't work, shortcuts don't work well, the menus are all broken (and the "OSX Menu" is that of the X11 application rather than the application's own), etc…

* X applications look horrible, even if you discount the clusterfuck that their interface might be, they redraw poorly and slowly, the areas "bounce" and the transitions tend to not "look right", and the whole widget set looks terribly out of place.

Compare: Gitk http://imgur.com/9mtGu.png, GitX http://imgur.com/xS2DI.png gitk looks like I'm back in 1993.

Ah yes, I keep saying "GUI" when I suppose I actually mean "look & feel".

Not sure if this is what the OP was getting at, but typing 'gitk' in my OSX terminal nets me "segmentation fault" and has for some time.