There's a lot to like about Java and C#, but here are a few things that Go improved upon that spring to mind. Note that in some cases, Java and C# may have caught up in the interim. Note also that some things (e.g., low-latency GC) understandably weren't available from day 0 in Go, but followed quickly:
* Static binaries by default
* Single, straightforward build system (no DSLs)
* Single, ubiquitous code formatter
* Minimal learning curve
* Zero-work package publishing
* Zero-work documentation generation and publication
* Testing framework out of the box
* Production-grade HTTP server out of the box
* Low latency GC out of the box
More generally, C# and Java have always seemed to have a philosophy of "make everything as abstract and configurable as possible, and make the user understand every configuration option in order to do anything (overwhelm the user with configuration)". Granted, I haven't used C# or Java seriously since Go was open sourced, so it's possible that these are among the things that improved in C# and Java following Go's release.
* Static binaries by default
* Single, straightforward build system (no DSLs)
* Single, ubiquitous code formatter
* Minimal learning curve
* Zero-work package publishing
* Zero-work documentation generation and publication
* Testing framework out of the box
* Production-grade HTTP server out of the box
* Low latency GC out of the box
More generally, C# and Java have always seemed to have a philosophy of "make everything as abstract and configurable as possible, and make the user understand every configuration option in order to do anything (overwhelm the user with configuration)". Granted, I haven't used C# or Java seriously since Go was open sourced, so it's possible that these are among the things that improved in C# and Java following Go's release.