I read an agent somewhere (I wish I could remember who, so I could link it) give the proportions.
it was something along the lines of:
20% sent in a form that makes it difficult to read. 6 point font, weird font, printed on both sides, pages dog-earred and held together by sellotape etc. This also includes people who fill the envelope with sparkles or somesuch to 'attract attention', or who use purple paper. Severe grammatical / spelling errors.
40% The prose itself in the first couple of pages. Every noun has at least one adjective, or the prose is so purple it blinds the agent, or the first three pages describe the weather, or mild to moderate spelling / grammar issues.
20 % plot is cliched, or is yet another novel about x, which has saturated the market over the last year, or plot is just, well, boring ( or boringly described) Or genre is not something this agent handles, or they just can't see where they will place a space opera about rampant lust bunnies from planet Zog.
10% The style. It's just not this agents cup of tea. Or it's trying to be experimental and actually comes across as unintentionally humourous, or makes the agent get the dictionary out three times on the first page, or the first sentence doesn't stop till page two.
Remaining 10% ( ish) will get partial / full request.