"Opinions may vary" is a common phrase in most form rejections.
While it's true, they do, sometimes the reason your query gets a pass is an opinion that's shared by a lot of folks. In other words, a whole lot closer to an objective assessment.
There's no standard that's 100% objective.
For every "you can't have a novel over 140,000 words" I'll show you six exceptions, and a lot of them published recently. But generally, excess word count is going to be a problem.
If I pass on your manuscript because there's no plot, that's something most agents would also notice. It's measurable. It's closer to objective than subjective.
If you think of subjective and objective as 1 and 10 on a number line (and here you thought you'd never need your 4th grade math!) with subjective at 1 and objective at 10, word count is somewhere near 9. Yes there are exceptions, but not a lot.
If I pass on your manuscript because you're a man writing about rape, that's entirely subjective, so it's closer to 1.
BUT a lot of women in publishing share that opinion, so maybe now it's closer to a 4 than it would have been 20 years ago.
Some things that are more objective than subjective: word count, tension, pacing.
Things that are entirely subjective: plot (what it is, not lack thereof); sympathetic characters; topic.
As a querying writer, you want to hone your skills in sussing out which is which.
Objective is something you want to pay attention to.
Subjective, not so much.
Why not pay attention to subjective?
Because one agent does not a market make.
Things I wouldn't read if you duct taped me to a chair and force fed me kale are sometimes books that enjoy a robust presence on the New York Times bestseller list.
What I want to read and acquire is just that: what *I* want.
I'm not the only agent in the world, or New York City, or even in this room as we speak.
Query widely, but pay attention if someone says you've got problems that are high on the objective number line.