I can't argue with most of those. The designer sucks, and I blame that for lots of the VS slowness. I never open XAML files in the designer. Not sure about 'scaling up' relative to GDI - I guess if you're a gun GDI programmer you can probably make it do pretty much anything, but I felt more productive doing graphics stuff in WPF - seemed to let you do quite a few cool things pretty easily. ILMerge thing is a pain, but not a major one (unless you've gone out and built thousands of assemblies and are getting slammed by load times, in which case you kind of painted yourself into a corner there). When you say "Virtually impossible to produce a scalable composite application." do you mean scaling development, or run-time scalability?
I can't help but feel if MS had paid more attention to perf (maybe re-platform it on top of Direct2D in the .NET 4 timeframe, instead of going all in on WinRT) things would be a lot better.
Anyway, thanks for sharing those pain points.