XP has a very dumbed down interface (I am not talking about the default theme), making it harder to administer your computer than in 2000. A lot of settings windows are obfuscated with "user friendly" crap that doesn't make it any easier to use for the non-technical Users (as they will not understand what they are doing either way) and just gets in the way for technical Users. XP Home is far worse in this regard than XP Pro, as fewer of the "user friendly" interfaces can be disabled. Security settings under Home are few and far between. You can not control access to shared files very well, you can not secure files from non-administrator Users very well, etc. Networking in Home is just a joke... Of course, this doesn't matter much if you only have a single PC in your house and never go to lan parties or anything...
"Each interactive user in XP Home is assumed to be a member of the Owners local group, which is the Windows XP equivalent of the Windows 2000 Administrator account"
source
This is a very scary prospsect... Running as administrator for daily use is generally a terrible idea, as not doing so prevents a lot of malware from being able to install itself (since you don't have direct permission to install software).
Product activation is definitely a good reason not to buy XP of any sort...
XP isn't worth the cost to purchase in the first place, but if you really must do so, at least get the Pro Version...