Enjoying all the thoughtful replies.
For me, if I will be in the trees or able to retreat to trees, the essential emergency items break down to wind shelter (tarp or 2-person bivy), bombproof ability to make and sustain fire, and the ability to heat water (for drinking and for pouring into a Nalgene bottle to make a hot water bottle that tucks under clothes). If there's a chance I might get stuck above timberline, then wind shelter, and a Jetboil-type stove with adequate fuel are my additional essentials.
Being old school and something of a Luddite, I'm somewhat irrationally resistant to communication devices (they change elements of self-reliance and thorough planning). That being said, lots of bad decisions are made because people are pressed to get home on time so as not to worry/anger loved ones or so as not to have loved ones launch unnecessary rescues. The ability of the Spot or the InReach to send a preset message saying you are fine but delayed is a huge benefit when a day goes a little south. The Luddite in me also admits the ability to call in a rescue is a huge benefit if the day goes a lot south (injury). If you are thinking of investing in one of these satellite communicators, the InReach is much superior -- as Kyle says the 2-way texting ability is a very significant benefit and its use of the Iridium satellite network gives better world-wide coverage (or so my research says).
Regarding the repair kit question that PhilH poses, to his list I would add two 3-foot Arno straps (very light and for some applications better than Voile straps)--these are in addition to the Voile straps. I also carry a pole repair kit (cut a 5-inch length of ski pole longitudinally so you can clamshell it around a broken pole as a splint and tape it in place), a spare ski-pole basket, and a few oversized screws for my bindings should I rip off a toe or heel piece (rare but it happens, especially with super light skis).
