When I'm commenting on someone's writing, one of the most common things I find myself saying is, 'I think you need either more of this, or less of it.' It might be some character who's characterised in such detail that they seem to be taking up an immoderate amount of space: readers expect the time they spend understanding and living alongside a character to be proportionate to their role in the plot, and can spend the whole book wondering when the person who took up the whole first chapter is actually going to return and blow up the bus. Which is fine, if they actually do... It might be a scene which goes on too long each side of the important moment: is its importance not written out fully enough, or is it actually not all that important and could go, its necessary elements distributed among other scenes? It might be some really painful, serious thing which just pops in for a bit: the death of a child, or holocaust survival, say, can seem crude, even callous, to have as a side-issue: a bit of instant angst, a plot device. Again it's a question of proportion: such things loom too large in us all to be used as a small piece of plot, and if such a big issue isn't given its due, at least subconsciously readers feel they're being manipulated by such a cheap effort at profundity.
When I say things like, 'Either more, or less,' I don't know which will turn out be right: the writer must decide for themselves. In fact I very rarely have the answer to what I think isn't working in someone's novel. What I have (I hope) is a good eye for what's not working and (I hope even more) a clear way of unpicking it to the point of pinning down the cause of the failure of tension/language/character or whatever. Another frequent phrase in my reports is, 'You've given yourself a built-in difficulty, here...'. When we're discussing it, the writer will say, 'What I was trying to do was...' That's usually the breakthrough point. Because then they can start seeing what to write and how to write it instead, to make what they were trying to do actually happen for the reader.
I often know what I would do, in my book, if I'd landed myself in such a problem. Occasionally I'll throw out some suggestions, though always as an illustration of my point more than a prescription. It's not my book, it's theirs, and their solution will be different. If the grand confrontational scene between Him and Her isn't working, I might shut them in a car together so they can't escape till they either fall in love or or drive over a cliff, Writer B might set it all in the middle of a hideous family dinner for twenty with uncles and cousins chipping in to make things worse and the turkey going up in flames, while Writer C might make Him (or Her) run out of the house and take the first tramp steamer for Buenos Aires.
What it comes down to is that even a cold, hard 3,000 word report is as much about process as it is about product, about problem-finding more than problem-solving, about why it's come out, not-quite-working, as it has. Compared to that, a few lines about whether agents and editors are likely to fall in love with this book, is the least of it. Once the writer understands why what they wanted to happen in the book isn't quite happening, they can see how to make it happen, and make it work. That's the book that agents and editors will fall in love with.