I agree that 'context' and passing this context between middleware is the biggest missing item from the standard http+"pat".
This library (Goji) solves it with a map[string]interface{}. This works, but has the downside that you need to type-assert your values from this map if you want to use them.
I wrote a library a while ago (http://www.github.com/gocraft/web) that uses reflection so that your contexts can be strongly typed. Martini offers a similar approach.