@maxdancona,
The thing that I ran into recently was there was not an option to bring a carryon. It would be one thing if say I had to pay extra to bring it on but they did not even have that option. Now it is quite possible that I could figure out how to purchase the next grade higher - but it became quite a pain in the a$$ to figure this all out - I just opted for United instead since I did not have to try to find that option.
It is as much as trying to navigate around the website to figure out what you really want (whether it costs more or not) and what is allowed. It was not until somewhat recently that this "no carryon" was even a different price. I feel the rules change daily.
Other things that cost more (depending on the airline) - booking a seat (nothing to do with extra legroom) some airlines do not allow you to book your seat and they assign it at check in; extra legroom; desirable seat location on plane; boarding order...not sure what else.
To me it is the difficulty of knowing what costs more and what is actually included when you purchase. It is kind of like hotels that are now adding in extra costs that you do not have a choice in. Fortunately the hotels stuff is getting ironed out where you know have the option (on most websites) to select the overall costs.
The problem I am having with the airlines are things they are not clear is not included or not allowed. It is more that they are not clear with these.