Travel etiquette is lacking these days. With airlines charging fees for checked bags, many people are carrying more than their allotted amounts onto planes, and airline attendants are not doing a good job of regulating this. I find it quite frustrating also when people will put a small bag in the overhead compartment, taking up valuable room for larger bags, when that small bag clearly would fit underneath the seat in front of them. It frustrates me when my rollerbag doesn't make it on the flight because the attendants do not enforce this rule.
I also get frustrated when I get stuck next to people who are clearly too large for their own seats. I paid for my seat, and I should be able to use the whole thing.