[joshvm]

Qt as an ecosystem is fairly complete for general application development. It includes, among other things, concurrency primitives (QtConcurrent) that are trivial to use*, sockets, database support, serial port drivers and charting (qwt for example). Plus lots of useful tidbits I keep finding like accessing standard system temp directories, making temporary files and adding registry keys (or equivalent).

I've never used GTK in anger, but presumably there's some feature overlap. At times I've used Qt without any GUI components because it's so convenient (nicer than Boost for sure).

It also leverages native GUI frameworks so most applications will blend in with the platform you're writing for (last time I tried GTK this wasn't the default). It's easy to style with CSS if you need it.