Most useful "feature": the battery really lasts around 6+ hours, esp. if you're not using wi-fi. This made some long airplane flights bearable.
I used to dread taking my previous Macbook Pro to a coffee shop, travel, etc. because it was bulky, required always plugging in the charger, etc. No more. Since it's so accessible on the go, my productivity has increased and I can quickly code up ideas anywhere.
It did take a week to get used to the smaller pixel size on the 13" screen-that-fits-a-15" resolution.
Sorry for the fanboy post, but seriously, this machine made my life as a developer that much better.