Personally, (speaking before my own submission goes under the spotlight...) I feel like I've already learned a lot from Pop-up Subs about sharpening up my pitch in order to catch an agent's eye and what an agent looks for when they dive into those first crucial paragraphs.
@AgentPete reckons the majority of agents have made up their minds on whether they're going to ask for more after reading no more than 700 words, hence that number.
On the Pop-up Subs I've watched, some have started well, then faded after a strong opening, which suggests the writing needs more work generally. So 700 words can be a useful indicator as to whether the writer can sustain quality prose (or verse!) through an entire manuscript. There's no doubt a great idea is a selling point, but it's not enough on its own. That's probably why "more cowbell" is such a popular option from panellists - our interest has been piqued by an intriguing blurb, but it's clear from the writing that the MS isn't yet a marketable proposition. Redrafts are often required and Pete and the panellists will usually say if they think the author's got something, but it needs further development.
The biggest clue Pete gives us on Pop-up Subs about how to snag an agent is if, on the strength of the submission, they can answer YES to the question, "can I make money out of this?"
So...is the pitch/blurb something that would propel a reader from the teaser on the back cover/website to open the first page?
Are the first couple of pages enough to keep the reader hooked, so they're willing to spend some of their hard-earned cash on the book?
From the author's bio, do I think they're someone I could work with?
So...possible categories:
1. Commercial potential: I want to read more.
2. Could be a go-er, but needs MORE COWBELL
3. Not my cuppa (but could be someone else's)
4. Back to the drawing board / Chalk it up to experience