1. Using the character's name too often in the dialog. Think about it. When you're talking to someone, how often do you actually say his or her name. You don't need to use their names. People don't talk like that and repeat someone's name often in conversation. It bogs down your writing and pacing.
2. He said, she said -- no, no. If you have only two characters in a scene once you establish who has spoken first, no need to use "said" anymore unless you have specific purpose to express more than the statement of someone saying something. For example, she said with an angry look of outrage.
3. Repeating what you just described. Too often writers will show the character doing something and repeat in the dialog what the reader just read. Once you've established something about a scene, no need to rehash it in the dialog or even reference it again. It bogs down the pacing and bores the reader with the redundancy. For example, a character walks in the room and eats peanuts. Then you have the character say, "Hey Bob! I just ate peanuts."