It's a matter of taste, but i found Ruby syntax to be annoyingly inconsistent, and do |..| ... end being something that isn't quite a lambda a huge source of confusion.

also activerecord doing "trust me bro" things behind the scenes (like pluralization) drove me up the wall.

to be fair ecto does a small bit of this too, but at least it doesn't change spellings (so you can global search an identifier).