Agent Kristin Nelson posted
a good blog entry about writing mechanics. It includes a list of ten beginner's mistakes that frequently occur within the first couple pages of a manuscript, ones that might cause her to stop reading. It's a good list-- seeing several of those issues in the first page or so of a book would likely make me stop reading, too.
Due to my puny attention span, anything I read nowadays had better be 1) short or 2) damned interesting. (Preferably both.) If a book can hook me right away, I'm in for the long haul. I love books like that-- ones where my eyeballs get a wee bit dry because I forget to blink, and I stay up until 5 am because I want to read just one...more...chapter. But if the first page is a trainwreck, that doesn't really bode well for the rest of the story.
It's the same with fanfiction. Fanfics with author disclaimers like "I know the beginning is kinda boring, but it starts to get interesting in Chapter 7, I promise! :D :D" make me laugh and laugh... and then I go read something else, because, seriously? Six chapters of meh to get to the good stuff? SURELY YOU JEST.
Life is too short to waste on bad fanfic. (Unless you write it for fun, in which case I obviously have no grounds to criticize you.)