I believe we're not considering the very difficult complexities of the sytem. Things like HIPPA constraints, state to state and company to company data transfer interfaces that need to be properly built, usually between disparate technology systems, and the immense complexity of medical-related data structures.
I doubt the front-end web access has been much of a problem, most everything I've read says the problems occur with data-related processing initiated from the web interface, things like account creation and getting available plans for a given geographical area.
Politics caused some of the problems too, what with so many states refusing to participate in medicaid expansion, which pushe a much larger-that-expected number of users to the federal healthcare website.
I do find it entertaining to listen to all the non-tech talking heads in media spouting off about technical problems that they have no understanding of, giving simplistic "solutions" that have no chance of working. So for the amusement value alone, the healthcare.gov website is a rousing success