One side effect of an upwards stroke is that you'll normally only hit the highest couple of strings. This upwards stroke starts at the highest strings, and (as cyco130 said) is usually a lot weaker, which means that you'll only hit two or three strings at most. More than that will usually sound clumsy, unless you really intend to go for that particular sound. A good example of that is the opening chords to Pink Floyd's Welcome to the Machine. Anyone will notice that those sound fairly peculiar, and that's because they're struck upwards.
Since it's difficult to hit the lower sounding strings properly in an upwards stroke, you'll often see people striking every chord downwards, when the lower strings are most important. For this, a good example is Pennywise's Bro Hymn, a song that most people will immediately recognize. This song's riff consists entirely out of power chords, on the lower four strings, and each strike has almost the same force to it. The only way to achieve that sound is by striking every chord downwards, over and over again.