I was with my children at a four year-old's birthday party a few years ago when I met the grandfather of the birthday-boy. As it turned out, before retiring, the man had been on the admissions committee of a prestigious medical school for decades.
So, I asked him, "Looking back at all of those years of experience, what is the one thing you learned from interviewing medical students?"
The man chuckled and said, "They have no idea what they're getting themselves into."
In life, we never have any idea what we're getting ourselves into, but I think of this man every time I practice the question "What will you like least about being a doctor" or "...least about being a [insert your medical specialty here]" with my medical school and residency mentees, respectively. Saying you'll love everything about being a physician or psychiatrist or pediatrician or internist sounds disingenuous and naive. You need to show that you have some idea what you're getting yourself into.
Having said that, I would avoid tacky topics like money. And talking about how horrible night shifts are is not going to win you many points. But a sophisticated applicant can infer what the challenges will be in medicine or in her specialty and can express them with aplomb.
...As with everything, practice your answer in advance.