I love 'The Sopranos', but I f'ing hate it when Tony eats an ice cream sundae - and we have all seen him eat a few. He stirs and chops, and stirs and chops some more, before finally putting the loaded spoon in his mouth. BRB. I just looked up idiosyncrasy on the internet and I think his playing with the ice cream is an idiosyncrasy that he possesses and my dislike of him f'ing with his dessert too much is my idiosyncrasy. Always looks like a good sundae, though.
Another think I hate (an idiosyncrasy?) is when people cut up their spaghetti (or any long-noodled pasta) before they start eating. That is a put-off. You don't need to use a spoon (I don't), just twirl the pasta on your fork and shove it in your mouth. Don't get fork and knife in hand, and proceed to chop up the noodles before shovelling them in your mouth with your fork. There shouldn't be a knife at your place setting for pasta anyhow...well, maybe lasagna.
One last thing...learn how to hold/use a fork and knife properly. Incorrect cutlery usage is another huge put-off for me. To try and put the proper way to hold/use cutlery into words would be impossible; you need the visual. But, I will tell you this. Don't ever hold your fork (when cutting something, like a steak) like a person would hold a knife when they are going to stab someone - like in the movie 'Psycho'. It's not proper and makes an individual look like they weren't raised right.