Objective-C on Mac OS X
Apple speaks a lot about Objective-C being the #1 language for Mac OS X development, but Carbon seems to be of a higher priority. Things like XML parsing, Keychain, registering for global hot keys, etc are ONLY available in Carbon, and these are all things off the top of my drowsy morning head.
Apple ought to start doing things instead of speaking about it. They’ve dropped EOF from Objective-C, which was one of the best reasons to use it. AppKit still is very nice, but that doesn’t matter much when you want to do things involving, for example, XML, where the support is excellent for other languages.