The asymmetrical design of the analog sticks is based on ergonomics and the typical use case. The left stick is at the neutral position for your left thumb, while the buttons on the right are at the neutral position for your right thumb. The vast majority of the time, you will be using the left stick and right buttons, so it makes sense that they are positioned for the most comfortable thumb position.
If you were to make the sticks symmetrical, you would have one of your thumbs out of a neutral position for the majority of the time, which is likely to increase fatigue and possibly cause cramping.
I'm not arguing that the xbox controller is necessarily better than the others, just that there is solid reasoning behind the design. Personally I prefer it, but many others disagree.
For the record, Cyborg sells a reversible xbox controller for those that really hate this, so that you can have a symmetrical version.