Data structures usually,
If you want to write a linkedhashmap or a dictionary where the keys are autogenerated from the values or any kind of structure not built into the language you are forced to either roll a new structure for each set of types or cast everything to and from (empty)interfaces.
While it's still possible with the casting, it doesn't feel as nice as the rest of the language does.
The other case seems to be any kind of generic pattern with channels, it's possible to build a channel-pair which acts as a single channel with a buffer only bounded by memory, but you can only do it for a single type.
All of these are avoidable or maybe even inadvisable, but it still feels like a hole in the language when you go to do something which is common, useful and trivial in C++/java/C# and you have to use ugly hacks or deal with heavily reduced functionality, especially when the rest of the language is so nice.
While it's still possible with the casting, it doesn't feel as nice as the rest of the language does.
The other case seems to be any kind of generic pattern with channels, it's possible to build a channel-pair which acts as a single channel with a buffer only bounded by memory, but you can only do it for a single type.
All of these are avoidable or maybe even inadvisable, but it still feels like a hole in the language when you go to do something which is common, useful and trivial in C++/java/C# and you have to use ugly hacks or deal with heavily reduced functionality, especially when the rest of the language is so nice.