Actually, your app will be written in Carbon, as most of the cross-platform GUI toolkits simply wrap around it. It's good to hear that you plan to do extra work to make your app Mac-like, though, because you're going to need to do that. You can save yourself a little work by basing your UIs on other platforms around the one you've created on the Mac. Users of other platforms will usually tolerate this, whereas the reverse isn't always true.

There are quite a few zealots who avoid apps written in cross-platform toolkits on general principle, and there are even some who avoid apps written in anything but Cocoa. That said, there's not likely anything you'll be able to do to appease them. The best you can hope for is to not mention what toolkit you used. It's a shame that this would be necessary, but what can you do in the face of zealots? Fortunately their numbers are decreasing as time goes by, so it shouldn't be too much of a problem for all that much longer.

By the way, I do suggest that you learn Cocoa and play around with it, even if you don't plan to do much with it. Aside from being a good mental exercise, you can use it to get a feel for what Mac users will expect of your apps.