Stress
Despite holding a job all about arguing, big sums of money, and conflict, the arbitrator remains above it all, flying carefree on a drama-proof cloud, completely unaffected by the outcome of whatever dispute she's embroiled in.
If you can learn to detach yourself from the arguments of others—responding to he-said-she-said with "hmm, interesting" as opposed to "oh, just shut your trap already, geez"—then your life as a professional arbitrator should be fairly stress-free.
Your work schedule will be somewhat under your control, too, putting you in a good spot to increase relaxation levels as needed. This is especially true for those coming off the hectic schedule of a judge or lawyer. Working fewer hours means seeing a whole lot more of your family than you did before. Now, whether that actually subtracts from your stress, as opposed to adding to it, is sort of up to you.