Other companies like Sony or Asus are extremely annoying with dozens of "lines" of very similar products with confusing names. I didn't look at HP, but I remember it was weird, because they kept the Compaq brand so it was even worse.
Compare with OS X (ignoring OS X Server). There are only two versions, period: OS X and OS X Family Pack. How obvious is it what the difference is between the two? Very. You just count the number of computers in your household you want to install OS X on, and buy accordingly. No upgrade/full version worries, no "what does this have over the other one?" worries, no buying something you don't need...
Sure, OS X doesn't come in so many "flexible" configurations as Vista does, but the thing about it is that you could just do that on your own if it meant so much, and otherwise you get a default config that works for most people.