My greatest disappointment with C++ has to be the QT library. They went ahead and actually changed the language, but kept the most insane parts of it. Let's face it - if your dev environment reads in $FOO source files and spits out C++ source files to a C++ compiler, you may as well make your $FOO language something better than "C++ with extra #defines here and there" . What a missed opportunity by trolltech.
Qt does not change C++ language. Qt application code (C++ side) typically compiles with multiple C++ compilers, so not only is it C++, it is often made very compatible/portable C++.
Qt build process does not read in $FOO source files, it reads in C++ source files, and generates extra source files in addition to the original. The original source file stays there, and can do anything C++ can do. This is a rather critical point, all modern features of C++ are fully usable in Qt apps code, and also support from them tends to be added to Qt (like connecting Qt signals to C++ lambdas). Another very important aspect of being pure C++ is, you can use any C++ tools with Qt C++ code. Debuggers. Static analysis. Any IDE "intellisense" (though Qt Creator does have a few customized features specific to Qt code).
I am not sure what you actually mean when you imply that Qt should have dropped "most insane parts" of C++, but it sounds like you suggest that Qt should not be a UI framework, it should be a new language of its own (which would then be compiled to intermediate C++ code, which would then be compiled by C++ compiler to machine code, unless someone wrote an actual compiler backend for it)? Creating a new language is an order of magnitude bigger undertaking in all fronts, than just writing a framework for existing language. Language design is hard, and getting people to use new languages is hard. Also, Qt was originally "just a library", so if it changed to "not C++ any more" in some version, it wouldn't really be Qt any more anyway, it would need to be called something else (and see the new version not getting adopted, while old GPL code base would be forked).