Comfort is my #1. I gave away my gaming PC because being able to play games on the couch, or in my bed, or outside on the hammock is fantastic.
Next is portability. Not only for the above, but the ability to travel and everything without compromising my gaming ability is huge. It's helped my homebody self get out and visit more interesting places, since I have the comfort blanket of my games with me.
Controls are third. The medley of buttons handles so much, and the dual track pads are both versatile and make typing at reasonable speeds possible. Plus the ability to remap anything and to make virtual menus has made features that wouldn't be accessible on normal controllers very easy.