How do you define portable? If you mean the same code compiling and running out-of-the-box on Windows, Mac and Linux then actually my experience with Qt has been very positive in that regard. Those are the only platforms I develop for right now, so I can't ocmment about portability to e.g. iOS or Android (although I believe Qt ports are available for them).

Qt doesn't reuse the STL container types, it provides its own - which I find much more pleasant to work with than their STL counterparts. The STL puts a lot of effort into providing types of flexibility that are irrelevant or useless to me (e.g. allocators) and doesn't provide lots of simple things that I need every day (e.g. an indexOf method for std::vector, a contains method for std::set, or a toUpper/toLower method for std::string).