Over at Jezebel, Sadie says "A good personal essay should illuminate a larger truth through a specific story. A poor one is just a narcissist assuming her experience applies to the whole world."
Or, as I like to say, the second is no different than a Dear Diary entry, and like hearing about someone's dreams or family history, can be kind of boring.
The first kind makes you sit up and go "yes."
Edited: because I am a bad self-editor when in a rush. Sigh. Sorry.