Most obviously, the Aluminium unibody (which comes with a 3.3 mm thinner body). But for most people FW800 (which is easily twice as fast as USB2) might already easily be worth the extra money. For others, the thinner body together with very sturdy and (to most people) more elegant exterior might be worth it.
So £100 for a port, a slot and a nicer case? I can understand the appeal for some, but not the "everyone should obviously get the MBP" attitude. Those three things certainly aren't worth £100 to me, which makes the MB interesting.