* Disturbing pauses
* Unnecessarily long sentences
* Too little jokes
* Too much jokes
* Repetitions (this is actually really hard to do, people tend to think every single phrase has its own unique meaning, but actually...)
* Slides hard to read from afar
* Slides with too many text
* Slides with too many code
* Slides with no code
* Slides with no helper shapes like boxes, arrows, stressed words etc
Also you should prepare for a Q/A section and study for some probable questions you come up by yourself.
This comment looks like an advice for "How to talk?" but actually these are the things to be careful about before you start giving talks around.
Good luck! :)