I feel like "my favorite interpreted language is slow" is a bit of a cop-out. Granted, it's the first thing that comes to mind when I try to enumerate the things I dislike about working in Ruby, but it's not really an interesting answer.

I might point to simple aesthetic things like JSON-style hashes of symbols looking funny:

    example = { a: :b }
    => { a: :b }
    example[:a]
    => :b

Or maybe "the way it imports things mean I wind up grepping code / googling docs rather than just looking at the top of the calling file to see the origin of an imported object, and then sometimes I find myself over-stating the namespace for an object just so Ruby won't get it wrong". Compare to python:

   from big_library import named_method

I suppose it's a pretty good question and a reasonable proxy for "so you have actually worked in this language for many, many hours, right?"

This is a great question as long as the interviewer is prepared that the reply might not necessarily match his/her opinion. Otherwise, the interview might end up in a ugly discussion of two opinionated people.

EDIT: fixed stupid typo