With all the stretchy-techy fabrics and designs that are more athletic cut, and using zippers for ventilation and ease of dressing, it seems that the good ol' button down shirt takes a lesser place in ultralight apparel.
I like 'em. You can "roller-blind" the sleeves, leave the front fully open, or part way, tails in or out, and layer it up with ease. And they have *pockets* -- a place to stick your glasses, a bottle of bug juice, a compass, etc.
I've collected a few in my thrift store hunts and have tried Ex Officio, REI, Columbia, and Cabella's (suspiciously like Ex O's) shirts. My favorite of the moment is an REI nylon shirt with a cape back with mesh venting. It's on the order of their "Sahara" pants and shorts.
Of course I don't have the weight handy, but they aren't a lot heavier than a windshirt. It would be nice to see some of the manufacturer's use some of the windshirt fabrics for a more conventional shirt design.
Almost forgot-- I have a microfleece button down shirt (a Sportif) that I like a lot too-- really warm and being able to roll up the sleeves and open the front is perfect for adjusting temperature and ventilation.