Thanks for the great question suggestions so far!!! Is it considered a good idea to ask specific industry related questions, (ex. How do the new federal regulations affect the way your firm deals with issues in the area of X)?
That's fine -- just make sure you're interested in what you're asking.
A few questions I like to ask...
What has made you stick around for X years?
Do you have thoughts on this firm v. _____ (a firm you have an offer from)?
What distinguishes your firm from another _____ firm (big firm in Chicago, boutique, etc.)?
How would you describe the firm atmosphere?
Do you have a formal mentoring program?
How is work assigned?
What would I start out doing as a first-year associate?
What did you think of the summer program when you went through it?
Will I have real responsibility as a summer associate?
How much interaction will I get with clients as a first-year associate?
Ask about what interests you, and if they're particularly excited about a certain subject, explore it more.