How usable is Qt without its preprocessing step?

I don't consider it unreasonable that Qt requires a special pre-processing tool, considering how large and comprehensive of a library it is.

Other similarly comprehensive libraries such as Boost and GLib don't require special pre-processing tools but do make extensive use of the standard C preprocessor. Qt could have been implemented using only the C preprocessor, but by using its own special preprocessing tool, it can provide a cleaner syntax and avoid many of the pitfalls associated with C preprocessor macros.

As has been answered already, though, you can use Qt without moc, just not anything that requires signals and slots. Yes, this does include all of the GUI stuff, but Qt is not by any means just a GUI library.

Qt doesn't require the use of moc just to use it, it requires that usage if you create a subclass of QObject, and to declare signals and slots in your custom classes.

It's not unreasonable, moc provides features that C++ doesn't have, signals/slots, introspection, etc.

So, to do something minimally advanced, you WILL have to use the moc preprocessor. You either love it, or hate it.

It's completely usable now. The maintainer of moc has made an alternative with slightly more verbose syntax than ordinary Qt, but it uses standard C++14 so there's no extra step.

It's called 'Verdigris'

(as an aside, moc isn't really a preprocessing step so much as a code generator. The code you write is valid C++, and moc doesn't change any of it. It just generates extra C++ code for you.)