(1) Two - UA and AA.
(2) General ability to obtain mediocre (instead of horrific) customer service from the airline. All other perks - upgrades, lounge, bonus miles, etc. - are secondary.
(3) Yes - AA Lifetime Gold.
(4) Yes. UA matched my AA Gold this year. I didn't really want to abandon AA (I'd rather be chugging along towards Lifetime Plat), but I fly to the West Coast often these days, and there is no easy AA way to get there. DL and Frontier both failed to respond to my status match request. Only UA matched - I've since flown about 55,000 miles on them.