I tend to agree that simple generics would have made things a bit less annoying working with custom containers (BTW - I think the standard library should have a proper set type, it's silly that there are a zillion set implementations out there with slight variations).

But the things that bother me most about Go are actually different (I'm saying this being a big fan of Go and having written a few projects with it):

1. The GC is very simplistic and prevents building memory intensive services (at least not easily) without suffering long pauses.

2. The lack of any robust IDE. I'm mainly working with Sublime, and used to work with the IntelliJ plugin. Both are decent, but sublime has a half working gdb integration, and IntelliJ lacks one completely.

3. Community maturity. go-get is awesome, and there are tons of libraries for anything - but most are immature and/or not maintained. Usually you have a problem and find 2-3 projects that look promising that already solve it, but then one hasn't been updated for 2 years, the other looks awesome but is 2 months old and incomplete, etc etc. It will get there, I'm sure, but currently there is an inflation of toy projects, and no mechanism to find the prominent projects quickly (I think pypi's keyword based approach helps with this a bit, BTW).