Honestly the only time I've heard passengers complain has been over things they weren't entitled to in the first place. Things like a free upgrade or free food or drinks just because they thought they were nice and special and deserved them.
I've yet to hear a complaint over something legitimate. But heard plenty by people claiming they were discriminated against over the size of their bag, the fact they required an extension seatbelt or the requirement to turn off their mobile phones. None of these things are examples of bad service by the airline. They are all examples of passengers somehow feeling that the rules which apply to everyone, don't apply to them.
And its just human nature for people to expect service to improve exponentially every year. There can be no improvement in service levels (or degradation or it) and yet complaints will increase merely because the service on offer is no longer meeting their expectations.