I chose airlines based on schedule, fare, and upgrade availability. I don't fly with the intention to gather more FF miles, except loyality does pay off for upgrades. If an airline mistreats me severely and the matter isn't resolved, this affects my airline choice. This has only happened with one airline. Service does matter somewhat. For example, I'll choose a NW morning coach flight because those ala carte breakfasts are good (better than CO, US to name a few). Actually, safety is my number 1 priority, but this is hard to judge. To me, it means avoiding certain planes (Embraer planes, ATR42, ETOPS routes using older planes) and certain airlines.