Apparent answer: aeration (mostly).<p>Also, sucking a straw forces you to drink in a way that maximizes exposure to your palate, somewhat the same way "pro" wine tasters drink wine.<p>If you have an hour or two free, get one of those little wine aerators and do the taste test. It's a night-and-day difference.<p>This also appears to be one of the topics addressed in Myrhvold's epic _Modernist Cuisine_.
<i>And that, in a nutshell, is why some things taste better through a straw.</i><p>It doesn't explain why some things don't taste better through a straw like ice tea or beer.