If you can't explain it, how are we supposed to state our opinions on whatever we're supposed to be talking about?
I think that it's been nailed on the head though by j79zlr, XP is nearly 4 years old, where-as Linux - dependant on the distro - is being actively maintained, tweaked, and most importantly, put in the hands of the public, nearly every day.
The users themselves control how linux looks, Microsoft have a tighter reign on the way Windows works, looks and runs, even if suggestions via their users are taken, there are far too many people to satisfy (around 90% of all computer users on the entire planet!) therefore someone out there may think that linux looks like the back end of a giraffe, and Windows XP is rather pretty.
Either way, all it comes down to is opinion, and we all know what opinion is like
It really depends. Personally, I think Gnome, which is what Ubuntu uses by default, looks pretty nice. It has clean lines, a good color scheme and looks nice and professional overall. KDE, on the other hand, which many other Linux distros use still looks like it was designed by pre-schoolers for pre-schoolers. I just find it very tacky, even though it may very well be "constantly evolving."
On the other side, I'm not a big fan of Luna (especially the blue scheme). Energy Blue (also known as Royale sometimes), on the other hand, although very similar to Luna in terms of the basic structure, is much more pleasing in terms of colors. And with the skinning community that's been built around XP's theming engine, you can make it look really nice too.
Another thing to consider is the fact that you've been using XP for several years now and have just started using Ubuntu. It is possible that you find the Ubuntu look-and-feel more pleasing simply because you're bored of XP's UI after using it for so long. That tends to happen very often. A two year old gadget (cellphone, mp3 player etc.) simply doesn't get you as excited now as it did the first time you used it. I remember the kind of buzz the XP UI generated back in 2001, simply because it was so different from the earlier "battleship grey" look. Just the fact that it was different made people like it, at least while it was still brand new.
Netryder, you just made all the sense in the world. I guess that's it, just bored of Windows UI, I have been using windows for ummm I don't know how friggin long now. I have used Linux before, a year ago I tryed Redhat, but yea I of course went back to Windows. Now Ubuntu, I just love it.
Linux is so nice and smooth because it uses transparency/higher res graphics and if it didn't look so good then hardly anybody would use it... Graphics wise/ that is what seperates linux/mac from crappy sadly more compatible windows....