This topic has needed to be tackled for some time. I've come across a few blog posts that have touched on it but they haven't gone beyond naming one thing they think shouldn't be tweeted. So here's my laundry list of things you just shouldn't be tweeting.
1.) Nothing Personal
I can't stress this enough. Twitter wasn't created to be a personal diary. It's a tool that you use to positively interact with people. Not tell them all your dirty details. So please, please save us the gory details on how your boyfriend broke up with you.
2.) Negative Stuff/Constant Complaining
I see this far too often in my twitter stream. And I am often tempted to de-follow the negative naysayer yet I'm afraid their fragile state of mind will cause them to do something drastic once they've discovered they're out one follower. Please keep your tweets as upbeat as possible, you'll gain more followers and your positive tweets will make you feel better. Guaranteed.
3.) Negative Stuff on Your Employer
Today's technology makes anything accessible; including past things you've written about your employers, coworkers, friends, relatives, etc. So make sure whatever you say/write is something you would feel comfortable telling to their face. Examples: Ketchum VP James Andrews tweet and @theconner's Cisco tweet.
4.) Your Location
This one should be used wisely (and in my opinion, sparingly). If you're at a conference and want to meet up with people on twitter then by all means do so. But do so knowing that if you do have a stalker then you're making it very easy for him/her to stalk you. So enlighten those on twitter to your location as long as you are well prepared (hint: mace).
5.) Pointless Nonsense
I really shouldn't have to stress this one, yet I see this all the time. I don't know who said it first, but one should keep this in mind when twittering: "Just cause it's in your brain, doesn't mean you should output to twitter..." If this is a problem for you, insert a filter in your brain and use it, because no one really cares about combining Pepsi, chocolate, bananas, and peppermint and tasting it.
6.) Incessant Conversations
Twitter was not meant to be used like an IM. Reply only once or twice then switch to DM. It is annoying for everybody else that is following you to know the details of your conversation.
7.) Anything in a Hospital
HIPPA's (Health Insurance Portability and Accountability Act) Privacy Rule ensures the confidentiality of communications with individuals/patients. I highly discourage a person to tweet anything that is overheard in hospitals because of that law. People's privacy are most protected within their personal homes and in their hospital room, breaching that...well it could be problematic for you.
8.) Anything That Goes on in The Bedroom
And speaking of bedroom.....I can't say I have seen anything regarding this topic tweeted, but I feel like I should cover this ground because I hope to keep it like that. What goes on in your bedroom should stay in your bedroom. Announcing what goes on behind closed doors could ruin your shot at becoming the next President.
9.) Anything Illegal
You'd have to be an idiot to tweet this, but well not everyone in the human race has an IQ of 170. So if you did do something illegal don't tweet it. Unless of course you want to get caught by the police and live off taxpayers money for the net 10 years...then by all means.