Maybe. But if the HIG is not that important anymore, I think that should also imply that Apple should allow for other UI toolkits in its stores, such as Qt.
This idea disappoints me. The system-wide Control-Command-D dictionary works in Twitter for Mac: That doesn't work in apps written in almost-facsimiles of Cocoa. That's only one example, but these's always something quirky about these replicas which is never desirable.
The average user would not be able to tell the difference between a well-adapting Qt application and a Cocoa application. We have a Qt application for searching treebanks, and when showing it to another Mac user, he replied "nice, you wrote it with Cocoa?".
Twitter's (or the App Store's) deviation from the Mac look and feel, on the other hand is very noticeable. And I am not sure it is proven that we are talking about "extending nicely into a different style of UI". The Twitter UI is plainly confusing (where do you drag this window?) and having the backward/forward buttons next to the window controls is questionable at the very least.
Did you just show it to the Mac user, or did you let him actually sit down and use it? Qt does a great job of mimicking the look of Cocoa apps, but it doesn't always get the behavior right, and if you're going to behave differently, you ought to warn the user by looking different.
Do the Mac App Store guidelines actually disallow Qt apps?
If I'm reading this correctly ( http://pastie.org/1236378 ), the only major requirements are that apps don't screw around with the HIG too much, and they don't use "deprecated or optionally installed technologies (e.g., Java, Rosetta)". Admittedly, the only Qt app I recall using on OS X is Last.fm, but that seemed to be mostly HIG-compliant (a couple things felt a tad out of place, but nothing major).
