For me it is probably this.
It works - easy as pie. No problems building, linking or worrying about dependencies. (it should be noted that this is a common property of hobby projects)
It's fast, which is nice for the small boy (or girl) in all of us. It is gratifying to make software that can run like blazes.
I get the control over memory layout that I really want. By day I am a Java dev, by night I used to hack Erlang. Neither of these allows me to lay out an array of structs contiguously in memory. I did this for a quad-tree implementation and got 2X speed up on read operations. This means that all the stuff I am reading about CPU architecture, cache-lines and latency etc. can be experimented with in an easy to use environment. That's just pure fun and gratification.
Comprehension. In Java I recently wrote a SOAP server (I know, lame) and it took me an hour of XML fiddling before a more informed person told me that we do that with annotations now.
"Which annotations?
These magical ones.
Ah thanks"
That is a server in Java. At the end I was as ignorant as when I began. But I had the annotations and I had a server. This is bullshit. Programming without learning is like eating without tasting anything. In Go I am learning as I code - this is ecstasy. When I am confused I can jump straight into the library source code, ever tried that in a JEE server? It is painful.
Websockets are fun. Go supports websockets in the easiest way I can imagine.
Errors. Explicit errors in Go are just great. Verbose, yes. But, the clarity of this approach surprised me. All the networking code I have written in Java seemed mysterious. I would hack away and end up with a block of code that would fail from 'somewhere' inside. Errors have taught me a great deal about the different ways my networking code can fail. This is possibly my favourite part of Go (which makes me sound a bit boring).
Green threads are great. I'm sure plenty has been said about this. And message passing too. I won't say more about that.
Closures are marvellous. I have to say that I had a lot of reservations about closures in an imperative language. But if you are careful they are as safe as anything else (in that soup of mutable state). And so much fun.
It should be noted that a lot of my complaints about Java can be remedied. I can read a book about the latest JEE annotations (I did start reading one). I could study the Java networking code better and learn about things that way. But the crucial point is that Go allows me to do all of this while I do what I like best. Hacking. I can build systems and learn at the same time.