I think one that everyone would agree on is eating with your mouth open.
Some others that come to mind are:
- Trying to talk while eating
- Slurping your soup or drinks
- Checking your phone often while at a meal (personal opinion)
- Burping (can be justified and of course sometimes happens accidentally)
There is also an old-fashioned one, which is not to put your elbows on the table while eating. I don't know anybody who still feels that way who isn't over 60 though.