Currently Qt requires copyright assignment (as I understand it) for code to become part of Qt proper. This is going to be a non-starter for a lot of open source folk. As I understand it,this was one of the biggest issues with the OpenOffice.org project in terms of community health, and one of the main drivers for LibreOffice
It's not just the copyright assignment: it's also the fact that Qt is now controlled by a huge organization (much like OO.o is). Nokias goals for Qt may already be quite different to KDEs goals for kdelibs, and if something is certain it's that corporate interests change. We cannot tell what Nokia wants to do with Qt next year, or in in five years.