- export templates is the canonical one.
- Universal references are a great feature in principle, but the way they are integrated in the language is far from ideal.
- Both features are great in isolation, but the interaction between initializer lists and aggregate initialization is a giant footgun.
- Coroutines are overly complex and still incomplete but I still have hope.
- Modules feel DOA so far.
- Unrestricted compile time evaluation is great, but the constexpr qualifier per se doesn't guarantee any useful property.
edit: overall I'm happy with the evolution of the language, but the standardization process has flaws