I get this all the time. Doesn't matter what goes on the table, one of them will always complain about something. I generally ignore it as they usually eat it anyway. There are far too many other more important things that I pick them up on, so I let this go otherwise I'd be a constant nag (DH probably would say I am ). If they are cheeky and answer back, I react - one DD was sent off to her room this morning for something of that kind. It's more difficult when they're step-kids - not being in that situation I don't know how I would react. I always feel bad when I'm pulling my kids up for sloppy table manners when they're are eating with their cousins who have far worse manners (and I'm itching to tell them off too).

Moaning over the food you prepare? I'd tell him how rude it was and that you've worked hard to provide nice meals so you expect if not gratitude, not to be criticised.
I won't negotiate on anything to do with their physical safety (seatbelts, cycle helmets etc), I don't allow violent games or films, I insist on good manners and generally encourage kindness.
I wouldn't tolerate swearing, aggression or insults.
Laziness gets on my wick but I try and negotiate terms and conditions eg tidy your room and you get your pocket money.
Everything else I try not to get worked up over.