Dell buys in huge bulk, they also sometimes buy second-bin components (such as the underclocked 6800 GPUs). They also base their motherboards on Intel reference designs for best compability, but they also tend to leave out certain features and expansion areas that we've come to expect from companies like ABIT and ASUS.
But their size also means that they're able to do a lot more testing and experimenting. When you order a Dell to your exact specs, you can almost be assured it will work perfectly out of the box. But it'll cost you, especially if you need some "advanced" features like DVI LCD rather than their "Free LCD Upgrade" which uses plain VGA, not DVI.
I also think the cashier at my local computer parts store is much hotter than the Dellf.