The best explanation that's been given to me is heat. Yeah, I know, it sounds weird since you're just gonna pour boiling water on the stuff. But the fact is that the blade type "grinders" are at such high rpm's that the blades will actually burn some of the oils, etc that give coffee its great flavor. A burr grinder, on the other hand, is much lower rpm, and so the grounds don't get burned in their short trip through the grinding mechanism. And it's not much, but when you're dealing with aromatic oils, such as in coffee, just a little bit of burned oil is enough to make the whole pot taste a little bit different.
And this is even more true if you're grinding the beans super fine. on my burr grinder, I just set the grinder setting to the required coarseness, and the beans don't really spend any more time being ground up, it's just the distance between the disks is reduced. In the case of a blade type, you have to keep grinding until the beans are the desired consistency. The longer those blades are whacking at the beans, the more heat is generated, and the more the flavor changes.
