If you made a list of the reasons I pass on queries, weak writing is right there at the top.
I see it all the time.
A colleague's recent blog post about a fellow who objected to her not reading the whole book to make a "fair assessment" drew the ire of the writer, who wrote a comment on the post that showed me in no uncertain terms that his writing was, in fact, very weak.
People who write well in novels also write well in the comment section. And in Facebook posts. And Tweet streams. And they write good queries. They may struggle like hell; they may fuss over commas and whether to invest in a font app that includes an interrobang (yes you should if only so I can get that query!) but they know how to string sentences together in a way that leaves me thinking "there's a writer." It may not be a book I want to read, but the writing is good.
So, now that I've terrified every single one of you, (and isn't that a good day's work!) how do you know if this applies to you.
It's really really hard.
The only way I know to have confidence in your writing is to learn to recognize good writing. And you learn to recognize it by reading it.
Thus: read books that really hold your attention. These may not be the same books people say are "beautifully written." And forget "bestselling" cause that will get you James Patterson and Dan Brown. All due respect to those gents cause they know their readers, and they have the sales to prove it, but these guys are not the standard by which to measure good writing. They're not BAD writers, they know how to write, but that writing isn't going to get you out of the slush pile and onto my desktop in this day and age.
So: read. That's the first thing.
And the second is: write. Write as much as you can. You get better with practice. You don't have to be working on your novel to practice writing. Write in a journal, write a pithy comment, write a scorching letter of complaint to me (I'm sure you'll think of something) all count as writing practice.
Write for contests like we have here on the blog. Write for lots of other contests too. Get feedback (or listen to the vast silence and know you're NOT getting noticed.)
Keep a writing journal. Analyze a book closely: what makes a paragraph flow? If you can't see it, you can't write it.
And don't forget to look back at your earlier writing. When you look at it now, can you see ways to improve it? Did you think the manuscript was pretty good when you finished it? Can you make it better now? Obviously, this is easier to do with short form work (like blog posts).
One of the things that helped me was Chum Bucket. When I actually had to write a specific response to a writer about what worked (or didn't) it forced me to focus and think, and try to write very clearly.
You can run your own version of Chum Bucket here: when the next writing contest comes along analyze each entry to make your own assessment of the writing. And don't just look at the entry and think about it. WRITE the assessment. Write as though someone on the other side of the correspondence is going to read it, and remember it.
Write your own book reviews even if you never post them. Write as though someone is going to publish it. Focus on clarity.
Going to a writing conference will not help. I will no more tell you that your writing sux at a conference than I will give you my whisky bottle and say "here, I don't need this anymore, I'm joining up with the WCTU."
What a writing conference will help you with is introducing you to fellow writers, some of whom may be good crit group partners for you, or who have reccs. on books with terrific writing. (Not everyone agrees on who should be on that list. MY list includes my clients of course.)
Bottom line: If you want to be a writer, I assume you want to be a good one. Improvement is learned. No one is born at the top of their game, not even Lee Child, not even Stephen King.