it is the first impression of the product. And lot much goes in designing an icon than it seems to our eyes. Some insights can be read here.
What happens when some familiar icons are intented to be changed? John Nack writes about the icon changes for upcoming Creative Suite 3 release of Adobe. It has same color scheme but different color and two letter from the product names as the icons. More popular product like Adobe Acrobat reader is out of the two letter category. Some likes it for its minimalistic and homogeneous approach. They say it is easy to differentiate out when you have hundreds of products from a company like adobe. And some hates it for not letting products retain their character.