One UX pet peeve of mine are tooltips which add nothing extra to the label or the purpose of the corresponding input field when I actually needed to know more. I have seen so many tooltips which are just a rewording of the label. A minimalist example is a label called 'user name' and its tooltip shows "This is your username". I know it's my username and not someone else username. The tooltip didn't add any value.

The tooltip should explain further what the input is used for, provide examples, specially if the entry needs to be in a certain format. In some cases explain what the different options are. I would have liked if DNN in the example below explained what the different options for user registration mean in a as few words, instead of rehashing the label. This way the user doesn't need to look them up in a manual or do a web search: