Be very careful when evaluating your boss—an increasingly common practice in many companies. Assume that he/she will find out who said what, even if comments are supposed to be kept confidential.
Also, give facts—not opinions—and do not repeat hearsay. Only mention things you personally heard or saw.
Avoid making recommendations even if asked what the boss could do to improve performance. Most bosses prefer to believe subordinates like and respect them. Anything negative you say could become a problem for you later.