I suggest you look back at some P4 reviews from when it first came out. Maybe even go back to really old Cumine reviews. (Aces.com is a good place to start looking.) Explaining it would take more time then I'm willing to give.
But simply put the P4 trades in every other advantage for clock speed. It takes twice the number of transistors, and twice the price then does the same thing as an Athlon with half the transistors at half the price.
(Which IMHO is why the P4 is crap even if it was a decent amount faster then the Athlon.)