I think watching the unaired pilot made me realize how essential Alyson Hannigan was to the success of Buffy the Vampire Slayer. The original Willow was just soooo wrong! You could say it was due to looks, since the original Willow is a little frumpy, and Alyson has a cute factor that makes her endearing and appealing (despite the horrible jumper she wears in the aired pilot). However, though appearance may be somewhat of a factor, I think it's clear after watching the two pilots that Alyson brings a spunk and humor to the character that the original Willow does not. Her facial expressions and line delivery just seem to light up every scene she is in, whereas the original Willow seems to fade into the background.
The original pilot, being shorter and without the real Willow, does not come close to the quality of the aired, hour-long pilot. That said, it's not awful either! It's just interesting to see what the Buffyverse would have been like without the real Willow!