Excellent feedback, as always!
Jay, I'm in the quality field (manufacturing, and specifically automotive). For a long time, I've enjoyed the ridiculous pace, travel, and intensity. I've travelled to some very cool places, helped solve some major puzzles, and gone toe-to-toe with the bigwigs during quality spills. It's just ... old. I'm 44, and I want to feel some passion for what I do. I know the new job ain't it either, but on the other hand, it will allow me to eat dinner with my family, won't have me losing sleep, and missing my kid's events at school. The benefits and retirement are fortunately a non-issue - spouse carries the insurance, and careful planning has taken care of the other. I doubt I'll even make to retirement if I stay in the field much longer; I've already gotten confirmation that the stress is not only fattening, but makes my thyroid freak out too.
Allison, I look forward most to getting home and pouring time into getting my gemologist certification (passion!) and building my new business. It's been on hold since I switched jobs, because I'm totally destroyed by the time I get home each night, and all I want to do is drink, or sleep, or drink then sleep. Truly, what I resent is every day that I'm not moving in the direction I want to go.
Jessica, your comment reminds me of my all-time favorite quote: "No one in the history of mankind has said on their deathbed, "I wish I'd spent more time in the office."

If I must be in an office for a while longer (kid in private school two more years, $$$), it makes a lot of sense to minimize the impact of it on the things I'd prefer to be doing. It'll still be quality, and still in automotive, but much, much less intense. More on the warranty end instead of on the front lines of production.
I just hate quitting, even if it's for a good reason. I'm a big people-pleaser, and this is going to be disappointing to my current employer. Not that I have any illusions that I'm not expendable ... I've suffered through enough of the downturns to see good people being treated like used tissues. Maybe that's the proper perspective: nobody's going to look out for me except me, so I might as well get on with it, hey?