The biggest thing I despise is repeating myself when it comes to validation and data-structures, and often using two different languages between the client and

The biggest thing I despise is repeating myself when it comes to validation and data-structures, and often using two different languages between the client and server means that's an unfortunate requirement. This can be mitigated somewhat, see JSON Schema and others, but most of them are leaky abstractions. Being able to truly, completely re-use data structures, validation logic, and pure functions between use-cases has saved a massive amount of time and pain across a number of production projects I've worked on in the last two years; Javascript being the language in question.