They're almost childlike in their simplicity, they're extremely distinct from one another, even at very low resolutions, and they only use a few colors. Obviously, the whole design is very web-savvy. Interesting that one of the industry pundits said (derisively) that they looked as if they had been drawn with a crayon... I took that as a compliment to the designers.
Here's a sampling:
My favorite of all of these is Outlook. Besides being a kind of neat program with a horrible interface, the small icon for the program evokes not just the "clock" element of the product's time-management features, but also consists of an O and an L. OL = Outlook. Cool!