This question is a tricky one because you do not want to foster an air of negativity during the interview. If you have heard negative things about the employer, you likely do not yet have the rapport to bring it up let alone help them make a move towards change. Keep your answer light and comfortable for the interviewer to address. The key here is to complete your response on a positive note.
"I read on Glassdoor.com that growth within your company can be slow. Could you comment on that for me? I am interested in joining an organization where growth is present when I prove myself to be a valuable team member. With that said, I have heard many more amazing things about your company, as opposed to negative."
"I cannot even remember where I heard it, but I did hear at one point that your hiring decisions can be a touch slow. Do you have a timeline in mind for filling this particular position? I am willing to be patient with your process, either way. It's an important decision, I realize."
"Negative does not always mean it's true - this is one lesson that I have learned after years of being in a leadership role. I have heard primarily great things about your company. Anything negative that I have heard, such as turnover or slow growth, are complaints that all enterprise-level organizations receive."
"I read a less than flattering review online from a disgruntled employee. I take those comments with a grain of salt, as there are always two sides to a story. When looking online, the positive far outweighed any negative comments."
"I have heard the usual comments that retail stores receive related to lack of training, unhappy customers, and high turnover. I asked around a bit before coming here today as I have a couple of friends who work here. Everyone I spoke to said it was a great environment, so I am going to lead with those positive thoughts instead!"
"One thing I have heard was that your sales targets are very challenging to achieve. Are you able to speak to that at all? I am a proven top performer who loves a challenge, but I certainly want to make sure that I am set up for success."
"Your school has a wonderful reputation. The only negative I have heard was in the district as a whole and was related to small budgets, and some additional cuts. I am happy to discuss this further if you are open to that."