People want to get what they're promised in a reasonable fashion. If the prices are hiding something like nonstandard seats or unreasonable baggage procedures, then that's a legitimate problem, not something they should shut up about because they should have known.
They deserve to know upfront how big the seat is, how big of a bag they can bring, and how much extra bags cost. On top of that, it can be useful if bag sizes and measurement have some standardization across the industry. Nothing major here or hard to provide.
Honestly, I think a lot is pretty well documented. But people don't have a good sense of what an inch more or less related to the seat actually means and don't necessarily know how picky a given airline is going to be on the size and weight of carry-ons. I expect that the carryon I routinely take on United might be an issue with budget European carriers.