MtGox customers surely ate not technical experts. Technical experts would hold their own wallet instead of handing it over to some guy in Japan who was publicly known to be unable of running a secure website, let alone a financial exchange.
Denying their technical expertise is more interesting. I've heard that theory often, and yet there are plenty of people I thought were technically proficient who lost money.
Indeed, I have some (small) amount of money in another Bitcoin exchange - it is convenient.
Perhaps the level of technical expertise of these users is roughly that of an average HN reader: ie, somewhat higher than the general population.