"Declarative" is a means to an end. Imperative gets you there at the same rate. Garbage collection compensates for inadequate resource management facilities. Currying is a neat trick that is fun when you find a way to make it do something useful.

Algebraic data types and GADTs are clever ideas very far removed from most real problems faced by real programmers, who have problems no language helps much with, and where the best we, as language designers, can do is try to keep the hell out of the way.

C++ has lots of such stuff too, mostly in template land. They can be useful to maximize the generality of libraries -- sometimes enabling great libraries -- but for most programmers most of the time they are at best a distraction, and more typically generate mysteries that absorb unbounded time.

Mark Dominus has some of the most insightful writing on the experience of coding Haskell.