Bad answer:

A lack of stability. I felt like the place could collapse around me at any time.

Try and stay away from anything that draws on the politics, culture or financial health of your previous employer. No matter how true it might be, comments like these will be construed as too negative. Also, you do not want to focus on a function that might be your responsibility in the next role. So think of something you disliked in your last job, but that you know for sure will not be part of this new role.

Good answer:

There was nothing about my last job that I hated but I guess there were some things I liked less than others. My previous role involved traveling at least twice a month. While I do love to travel, twice a month was a little exhausting, I did not like spending quite so much time out of the department. I am happy to see that this role involves a lot less travel.