Going from factory work to my chosen career path in computer graphics.
Sort of a no-brainer that I'd have to take a pay cut from a dirty, dangerous job in an iron foundry to an entry-level job where I sit at the computer all day and look at pretty pictures.
Although I'm almost back to where I was after less than a year.
The big downfall was that the factory job was almost a guaranteed 55+ hour a week job and once that overtime became standard, it was easy money. The new job is an average of 45 hours a week, so I really felt the difference when it changed.
If it was in the same geographic area, then maybe this question would mean something. If you are moving, like Rage said, from one coast to another, then the pay cut really doesn't have meaning unless you factor in cost of living.
1. better working conditions
2. to be able to spend more time outside of work than at work
3. upward mobility with new job
4. move out of a 'bad neighborhood'???
5. to be able to buy/afford house or a bigger/better house
6. reduce commute
7. prevent job 'burnout'
8. tired of shoveling snow; move to place with optimal climate/weather
9. potential high rewards; other incentives besides base salary with overall compensation coming out better.
20% when I took my current job. No more wasting 4-4.5 hours of my life each day commuting was the primary reason along with the fact that I was burned out on the vagaries of working on Federal Government contracts.
About 10K which was about 25% percent of my salary way back then. Went from a position with a good monthly incentive plan to one with a slightly higher base salary and nearly no incentive/bonus. Did it because it was a foot in the door and I was able to springboard it into something paying much more.
I took about a $20k cut in salary when I moved out of the IT business to take over the family business. Since then I have significantly increased my earnings and I am now earning far more than I could have hoped for if I had stayed where I was.