As a casual nim user, there's plenty I probably don't appreciate enough in there, but thing I'm most looking forward to are the default values for objects.
I am looking forward to them supporting distinct DateTime types (basically used-defined clones of the DateTime type with custom behaviour). Currently using the constructor package to do the same thing, but being able to throw out a dependency is always nice.
I suspect that is a headliner "syntax" feature for many more serious Nim users. Initialization, like error handling, is one of those things that is easy to mess up.