1) Fare
2) Type of aircraft (the bigger,the better)
3) The overall reputation of the airline (United=OK,Air Zimbabwe=Not OK)
4) The accent of the pilot (American,Canadian,British,Aussie are best)
The physical appearance,or the attire,of the stews mean nothing to me (I don't see an airplane as a singles' bar)
I got of a DC-9 when I saw the plane was 44 years old. I took another flight.
I got a little nervous once when technical manuals and maintenance logs were in loose leaf binders in an open bin in front of my seat, with some of them dumped on the floor. Then there was the pilot who looked to be about 19 (in fairness, he was south Asian and was probably at least 22) dressed like a WWI flying ace with leather jacket, jaunty cap, and scarf around his neck and dangling nearly to his knees. Flew with him through a typhoon on an airline that had just crashed three planes. Sheikh Abdullah al-Mohammed bin Muhammed with four wives and a dozen kids was ok, but the eight foot tall pile of baggage with live animals in wooden crates was a bit much. Of course, that was a clientele issue, not an airline issue. The airline handled it very professionally. I didn't have to fly with any goats or chickens in my lap.
I too try to be selective. Sometimes you get pinched.