I've never judged a book strictly by the first page except that if it's in first person, I put it down. If it's in present-tense, I THROW it down!

Second, I look to see if it starts with backstory and description. I put it down. That tells me the author has trouble getting to the point. The story doesn't have to start with a huge bang, but it should start with SOMETHING HAPPENING. SOMETHING. If it doesn't, that's a huge red flag for me. Then I look further to see if there's any empty space on the pages. If not, that tells me the author likes to ramble, another red flag. That means the story doesn't move, it more than likely crawls. As a writer AND A READER, I've seen it all. That's not only what I look for as a reader, but what I strive for as an author. To move the story along.

So, something has to happen on the first page, not necessarily a slam bang scene, but something to move the story forward. Description and backstory can be leaked in later in SMALL doses.