With all the amazing accomplishments people have been doing, it's only natural for the rest of us to think "It must not be all that hard. Maybe I can do it too."
I remember being pretty gung-ho about trying Winter hiking after reading the chapters on it in "Forest and Crag" — Gee! It sounds like a lot of fun! And easy! — but I haven't really done any after all. (My partner has asthma, so she can't breathe the cold air. That's it.)
Regarding the most recent tragedy, with the couple on Lafayette, it sounds as though there may have been a micro-climate situation there. A poster above mentions that he was on Flume at the time, and able to see the snow plume on Lafayette — a clear view where he was, a whiteout just a short distance away. Remember a few years ago, the couple that spent a night (it was the day after Thanksgiving, about 1998) in a snow cave on Lafayette after getting caught in a sudden snowsquall? My son and I (and dozens of other hikers) were on Welch and Dickey that day. It did not snow at all on Welch-Dickey. Again, a different condition just a short distance away. By coincidence that day, my son and I mistakenly got off the trail and spent more than a few anxious minutes trying to decide what we should do. If there had been a squall then and there, we would have been in deep trouble.
I imagine that the man who froze to death near South Twin probably heard the weather forecast and just thought, "Eh. I've been cold before." I regularly used to ride my bicycle 15 miles to work (from four to five a.m.) when the temperature was about 0 degrees Fahrenheit. Sure it was painful at first, but once I'd warmed up, after a mile or so, it was bearable, and after I'd arrived and suitably impressed everyone (a co-worker once exclaimed "You rode your bike today?!? You must be— unusual.") I'd wonder how low I could go. "Well, I made it at minus two, maybe it'll be three below tomorrow." I once went jogging at -19; by the time I got back home (three miles later), it was -22. Hiking in the mountains, rather than jogging in one's neighborhood, could easily put one beyond the point of no return. You think it can't possibly get any worse, and it does.