There's a massive amount of software that exists outside of note-taking apps where the customer will be more than happy to put up with different-looking buttons because the value the software provides is so high.
There are many software categories where visual-design decisions are not the only competition advantage.
Design is far more than just visual, and your dismissal of design as "different looking buttons" suggests that you don't really understand design very well.
I don't think anyone was, least of all me. In any case, I'll bite:
Different UI toolkits make doing certain things hard, certain things easy, and some things damn near impossible without large effort on the part of the developer.
So when the parent commenter says "Nobody on OS X wants a Qt app", I'd argue that yes, although you can make a "well designed QT app", it's much harder to make a "well designed OS X app" with QT.
Why will it be less likely to be a well designed OS X app? Because it will be different to a Cocoa app made with interface builder, in subtle and sometimes not so subtle ways. Sure you can code around these and make adjustments, but the level of effort and investment to get to the stage of if you'd just built it with a more suited tool for the job (Cocoa / Interface Builder on OS X) is quite high.
This is why there is some truth the the admittedly generalised statement made above that "Nobody on OS X wants a Qt app".
I think a more useful statement is that "Nobody on OSX wants a poorly designed app because the platform has a high standard for design". It is possible to design well designed apps without using Cocoa - see Sublime Text.
