For me and for many others that I have spoken with, the really scary part of using a large corporate website is that you'll go there for something very specific, such as a software patch, and emerge four hours later with nothing, having surfed in maddening circles, always seemingly two clicks away from your goal.
The problems faced by huge corporate websites such as Microsoft's or IBM's are many, but they are largely self-made, in my opinion.
Huge corporations are simply unable to do anything which they perceive may compromise their "corporate identity". Thus, they feel they have to attempt to incorporate and interconnect every aspect of their endeavors within the context of a single site. Of course, interconnectedness is one of the things that the internet is all about, but that does not mean that it is always desirable or useful to attempt to link everything together.
The use of corporate websites has evolved rapidly, and corporations have failed to address that evolution effectively. In many cases, what used to be merely a corporate showcase is now the primary point of service provision. Consider Microsoft and their many 'security enhancements', 'issue' fixes, driver updates, 'service packs', their web developer resources, their online technical support files, product reference guides, and so on. The problem is not that they provide these services online, it is that they attempt to provide them all within the same context. What about a 'microsoftDrivers.com' that contains only drivers, a 'microsoftSecurityUpdates.com' that contains only security updates? Unwieldy? Clumsy? Try spending four hours of your working day searching for a simple yet vital piece of information that you know exists somewhere on a corporate site: when you've emerged empty-handed, bug-eyed, and angry, we'll talk about it again.
Note: This writeup is not intended as an anti-Microsoft rant. I have chosen Microsoft's site as an example simply because it is such a clear illustration of corporate bloat.