Application programming in the scale of Photoshop, Word, etc. Video games. All kinds of multimedia apps and video/music editing apps.
C is too low level for those kinds of things, and Go too high level.
Plus it's not just the language, that's a mistake: it's the whole ecosystem that matters.
E.g you're gonna find more experienced C++ programmers to make you the next, say, Call of Duty or Logic Pro 9, than you're gonna find Go programmers. And far more code and libs to leverage.
And, I'll restate: from the 1000-or-so lines of Go I've written so far, I'm pretty clear that I wouldn't want to use it for frontend web stuff (anything with serverside templates). But I'd probably use it before I would use Node.js for a backend web service, or maybe even for a single-page app.
Outside of the lack of bindings to proper UI frameworks, I couldn't say why you shouldn't write e.g. Word in a higher level language than C++. You might have to be careful with memory usage in some components, maybe even write those in native code, but for the vast majority of interactions something higher level should work, shouldn't it?
After all you can implement something akin to Word in JS + HTML (Google Docs).
The Go team's idea was, look at languages like C++ and replace groups of individual features with orthogonal components that can be composed to similar effect.
Pike is a little dismissive of C++, as am I, but ultimately he acknowledges the truth of the matter: if you're dead set on writing programs using template generics and inheritance hierarchies, you're going to stick with C++. And that's OK. We don't need to argue about it.
I am very new to Go, have a background in C/C++, am Ruby today, and was Python from '02-'05. My take is that there are definitely still things I would write in C, and I would still write web frontend code in Ruby. But I see a place for Go (backend network code, network clients, things like that) and I personally see no place at all for C++ (but then, the C++ people might not see any place at all for C). Go doesn't have to be all things to all people.