The other reason: things inside maps are often pointer types with mutable data. Even if access to the map is properly locked, if two threads take a pointer out

The other reason: things inside maps are often pointer types with mutable data. Even if access to the map is properly locked, if two threads take a pointer out of it and directly or indirectly keep a reference (for example, re-slicing a slice without copying it), then one can stomp on the data the other is reading. The fixes for this are subtle and annoying. Either you have to lock around your data, make it somehow immutable, or force copying rather than referencing (by making it "value" data). Go would really benefit from a port of Clojure's data structures.