I haven't seen any cross-platform toolkits that abide by Mac standards. Funny enough, the closest is Mozilla's toolkit. Other than that, there are always telltale signs—e.g. no one seems to grok Mac's toolbars even though they're organized by the basic laws of proximity and grouping. Qt apps have the lazy grid toolbars just crammed from left to right, occasionally with extra-large icons for the ‘designy’ look, and always with colorful icons unless something app-specific is made (like in Calibre). Plus the noisy dividers.
Also, since Qt handles all the graphics and input, it has to reimplement everything high-level that the systems provide out of the box, and presumably to track changes in all of that. Hence the past bugs of one-pixel offsets between elements where they can't be in the system. And the present non-support for system-wide key rebinding on Mac—e.g., home/end instead of cmd-left/right.
But yeah, Qt is better than something like not-obsessively-tuned Swing. As for GTK, it seems to depend a lot on the author specifically not forgetting to bundle the native-look theme.