loss of carbonation: as with any carbonated beverage, beer will lose its carbonation over time. In other words, if you take too much time to drink your beer, the last bit left in your glass will be flat.
There is no scientific proof that pouring a beer one way or another will actually impact the taste of your beer, but pouring a beer the correct way will impact how you perceive that taste. That's because a beer's head holds (and releases) all of the aromas, and aromas are what help our brain perceive flavors.