I noticed an ad for a major airline selling the fact that they don't charge for (the first couple of pieces of) checked luggage. And as my premier status on various airlines ebbs, I find myself charged even for the first checked bag more and more often.
Despite persistent rumors that they will soon start, no airline that I know of is charging for carryon baggage. Here's why I think they've got that backwards: Charging for checked luggage encourages people to carry on more and larger bags.
So what, you say? It's their own burden to carry and stow the stuff. Perhaps so, but it also makes the boarding and deplaning processes much longer for everybody, and according to the Flight Attendants Union it also causes thousands of injuries a year to passengers and airline personnel. The very people who carry on a larger bag because they want to save time by not waiting for baggage claim are delaying everyone in boarding and everyone aft of them in deplaning.
When overhead storage space is free and unassigned (you have no particular claim to the space above, unlike the space under the seat in front of you which seems to be part of your tiny purchased real estate) you get the classic tragedy of the commons as there's no disincentive to overusing the resource.
In short, airlines are lax about enforcing size and quantity limits on carryon bags when a simple schedule of fees would take care of the problem for them. And the best part is, if you're too cheap to pay the additional carryon bag fee, you still reap the benefit of getting on and off the plane faster!