Every law firm or company has aspects that are less than desirable for some people. In fact, this may be the reason that someone is considering a move. However, it is never a good to dwell on this too much in an interview.
No one should tolerate an environment, attitude, or specific acts that are truly undesirable; and looking for another job is one course of action that can remedy the situation. Just keep in mind that sharing too much can actually hurt you because you can come across as a complainer or generally create an aura of negativity (even if it is not a fair assessment).
When asked why you are considering a move in an interview, and the reason is something undesirable about your current work, then share as much as necessary to answer the question and move on. Your answer should not describe anything in detail but essentially convey that it is not a good fit and that moving is best course of action. Yes, the interviewer might be intrigued by the dirty laundry but in the end it comes across unprofessional and otherwise negative. The key is turn a potential negative into a positive as quickly as possible – chalk it up to a learning experience, etc.