For me it was the constant roster changes that ruined any chance of any kind of personal life in the summer. On top of that the punitive "just culture" that meant most people spent more covering their arse than actually focussing on the job in hand. For others it was the prospect of being promoted out of base despite the fact that DEC's were being recruited in to their base. Despite guarantees that it wouldn't happen again it appears that it is again this year.
The constant fire fighting that was the roster disruption (not the fault of the crewing department but not helped either they certainly didn't care that your life was being thrown upside down again) meant I dreaded going to work because I really didn't want to see what was in store for me then. Calling friends/family whatever as soon as I reported "you know those plans we had, yeah I'm going to have to cancel again. Let's reschedule for November.
And yes people have gone to all of the airlines mentioned. Most have gone to BA, some to Thomson, a couple to the middle eastern airlines. I'd say about a quarter of my aquaintances that have left have gone to Ryanair.
Yes I will be worker a lot harder in my new employer but my roster is stable. I can actually plan a life outside of work. That'll do thanks.