Nil pointers are #1 on that list. Tony Hoare called them a "billion dollar mistake"[1]. In Go, as in C, C++, or Java, any pointer can be `nil` (similar to `NULL`) at any point in time. You essentially always have to check; you're constantly just one pointer dereference away from blowing up the program. Since ways of solving this problem are known, I find this hard to swallow in a new language.
Compare Rust where, unless you have explicitly used the unsafe features of the language, all pointers are guaranteed non-nil and valid. Instead of a function returning a nil pointer, it returns an `Option` type which is either `Some ptr` or `None`. The type system guarantees you have considered both possibilities so there's no such thing as a runtime null pointer dereference. Scala has a similar `Option` type, as does Haskell, calling it `Maybe`. In 2013 I don't want to still constantly check for a nil pointer, or have my program blow up at runtime if I forget.
The second disappointment is that when I looked into it, it seemed there are ways to call C functions from Go code, but no working way to call Go from C code. Maybe that wasn't a goal at Google, but it seems like a missed opportunity. As a result, you can't use Go to write an Nginx module, or an audio plugin for a C/C++ host app, or a library that can be used from multiple languages.
I think there is a real unmet need for a higher-level, safer language you can use to write libraries. Imagine if zlib, or libpng, or the reference implementations of codecs (Vorbis, Opus, VP8) could be written in something like Go. Or spelling correction libraries. Currently we have two tiers of libraries: those written in C/C++, which can be used from any language under the sun (Python bindings, Ruby bindings, Perl bindings, PHP bindings...), and those written in high-level dynamic languages (Python, Ruby, Perl, PHP, ...) which can only be used by the same language. We need a middle ground. C isn't expressive or safe enough to deserve to be the lingua franca like this. And Go is tantalizingly close to replacing it, but not quite.
[1]: http://qconlondon.com/london-2009/presentation/Null+Referenc...