You know this question will be asked at some stage, so have your answer ready in advance. The rule of thumb is to always remain positive about your current and previous employers because you never know when your paths may cross again. Besides, who are you going to turn to for a reference?

For example:

I learned a lot from my previous employer and enjoyed my time there. However, promotional opportunities were few and far between and I am keen to advance my career sooner rather than later.