The best way to answer that is to point out a legitimate concern that is being addressed.
Well, the furniture is dated, but we're getting it replaced next week
or
Well, our desktops are a bit slow, but starting next month they're replacing them with newer models"
Then carry on with something like...
But that doesn't bother me because the people are great, the working conditions are better than most, and we've got a really great benefits package.
Never stay on the negative a second longer than you have to.