Loud talking on phones/Skype + parents letting their children run around (cannot blame the kids, nor is it all of them) are my two worst. People taking up too many seats is definitely up there, as well - (usually) being a solo traveller I'm very conscious of that. Another runner up is any sort of laying on furniture/shoes off, but I'm sure others will disagree on the former aspect.
For the life of me, I cannot understand having a potentially (at least somewhat) confidential conversation at full volume in a very public space?
Edit: adding seeing people leave piles and piles of food/drink. I don't just mean one spoonful here or there, either. I understand it might not be great food, but I can assume many people doing this are regulars and know what they're up against? If not, I'd always 'try before you buy' at a buffet setup. A bit annoying given how lucky we are to have it (and how many people in the world don't) and that it ultimately drives costs.