"Microsoft's biggest and most dangerous contribution to the software industry may be the degree to which it has lowered user expectations."
- Esther Schindler, OS/2 Magazine
Windows Explorer is probably the most used file manager in the world. Given this, as well as it being such a central, oft-used application, you would probably expect that quite a bit of thought would have gone into it's design, utilizing mankind's extensive experience developing this type of application (e.g. by studying the finer points of the classic excellent file manager, Norton Commander), and that Microsoft would make some reasonable effort to ensure that this important application did it's job well.
If you expected that though, unfortunately you'd be flat out wrong. In the 6 years since Windows95 came out, not only has the badly designed, badly thought-out Windows Explorer scarcely improved at all, it's actually gained a large number of new bugs over the years; many of these, I presume, are from the "benefits of integrating the web browser into the operating system" that we keep hearing about.
Anyway, here are a small handful of the problems I've found with it.