I build desktop apps for the Mac but want to build cross-platform software going forward. A native look'n'feel is a bonus, as well as easy deployment. I also don't want to be tied down to proprietary APIs like I am on the Mac with Cocoa, Core(Whatever), etc.

From a language perspective I prefer Objective-C, C#, Vala over C++. Personal taste. Sadly those languages don't have bindings to the toolkit which seems to have the most momentum, QT. There are third-party C# bindings to QT, Qyoto, but that requires using Mono which for some Linux users is a no no no.