Officially, there aren't many rules on Twitter, but there are definitely a lot of best practices that can help you fit in to the community a little more easily. Think of it as Etiquette for Twitter.

Here are ten practices for savvy users to avoid (in no particular order):

Don't send automatic direct messages (DM) - people don't like them because they are impersonal.
Don't fill your feed with only broadcasts of your message - engage others. A good rule of thumb is that less than 1/3 of your feed should be new messages from you and the rest should be replies and retweets.
Don't call someone out for unfollowing you, most of the time it isn't personal. And often, there's a twitter bug. Politely ask if it's someone you regularly engage with, but don't make assumptions that it was intentional or that they will resume following.
If you're having a personal discussion with someone don't move their name to the middle or end of the tweet with every reply. By keeping it as the first thing in the tweet only people who follow you both will see the tweets. Those who see both sides of the conversation can follow along (if they want) without losing the context of what's being said.
Don't reply to every tweet by retweeting (RT) and commenting. This breaks the thread of the conversation and it can be frustrating to your followers when there isn't enough context to understand what is happening with a RT.
Don't beg people to follow you. People will generally follow you back if you engage with them in some way. It would be far more meaningful if you replied to something they were talking about instead of simply asking them to follow you.
Contrary to what some experts believe, you don't have to follow back everyone who follows you. Twitter is about creating your own unique user experience.
Don't use Twitter to complain about every bad experience you have with businesses, services or products - social media isn't meant to be a place to air every grievance. It doesn't look good when a business owner uses it that way.
Don't autofeed from other social networks unless you're sure your tweets won't be cut off. People don't want to have to go elsewhere to see what you're talking about. More importantly, it is obvious when you're feeding from other services and it can give the impression that you can't be bothered to take the time to tailor your message to more than one social network (because it doesn't have to take that much time).
It isn't necessary to thank everyone who follows you. If you want to make a point of acknowledging them, go read their profile and start a conversation. Make it natural. The thank you is a nice gesture, but it isn't impolite to skip it.

The most important things to remember are to be friendly, personable and add value. The practices above are "frowned upon" because they are perceived as not adding any great value. Spend your time finding other ways to connect so that followers stay actively engaged with you.

What other Twitter Don'ts would you add to this list?