As an interviewer, I have never had a problem answering this question honestly. Anyone who shows discomfort with the question would give me a bad feeling about the company I was interviewing.
It's perfectly normal to have things about your job you don't like. What's so bad about talking about them?
What if I (the interviewee) happens to know your boss, and your boss doesn't know you have issues with the work? That's why as an interviewer I'd tend to be circumspect.
I have no problem with anything I say during the course of conducting an interview getting back to any of (a) my manager, (b) HR, (c) Legal. None!
What's all this fear about the boss knowing you have issues with the job? Everyone, repeat, everyone has some issue, however minor, with a job. Owners, managers, etc. already know this. It's not something to hide unless you're working in a really sucky place that can't handle any level of dissent. And in that case you should probably think seriously about leaving (unless you like that kind of thing).
Wanna know what my answer is to most candidates who ask me about something I don't like about my job? The bureaucracy. We are in a highly regulated industry and it can take an annoying amount of time and paperwork to complete some tasks and it gets tedious and frustrating at times. There, I said it. Believe me when I say that the people I work for are well aware of this problem.
It's perfectly normal to have things about your job you don't like. What's so bad about talking about them?