- Go way over your allotted time to speak. Even if the chair of a session lets you do this, don't. It is rude to the audience and to other speakers.
- Lack empathy for your audience. Take a few minutes to imagine what the audience might want to get from your talk. Some of the speakers here are much more concerned with what they will get from the presentation.
- Use a lot of slides with way way way too small text or images.
- Answer cell phone calls in the middle of the audience. Yes, that's right, scientists can be jackasses. Imagine that.
- Corner people who are on the way to the restroom. Let people go.
- Make an opening statement when asking questions like "That was a great talk" or "That was an interesting talk" or anything like that. Don't be a suckup. Just ask your question.
- Be rude to the meeting helpers when you forget something. Come on. If you are not registered for the meeting it is most likely that you screwed something up, not the meeting.
- Ask many follow up questions after your question. If you want to have a discussion, buy a beer for someone. If you have a straightforward question that is answerable - ask it. If you want to make a simple statement, fine. If you want to go on and on ... get a room.
- Write in your blog in the back of the room (hey, I did not say I was perfect).
- Have too little time for breaks. The best part of conferences is the coffee and other breaks. No need to have too many talks. Have lots of breaks.
I am sure there are other things to not do ... but these are those that come to mind right now.