Package managers – your yums, portages, apts etc – must be one of the best features of linux. It used to be pretty impossible to keep an installation up-to-date, now it’s trivial most of the time.
I actually wonder if this is Linux’s killer feature – whilst your Windows installation slowly rots until you are prepared to face the pain of a major OS upgrade, your Linux installation has been incrementally improving itself. I reckon that Gnome overtook XP sometime in the last few releases in terms of features and (in some places) usability.
Sure, Windows will probably retake the lead with Windows 7, but how long will it be until the next major version of windows? XP is a staggering 8 years old, will it take as long until most people are prepared to update again? In that case, Microsoft may find they are in serious trouble by then, facing Linux distributions with with improved kernels, file systems and desktops, especially with distributions like Ubuntu pushing usability and user experience.