In the wheelhouse of writer as reader, a "good reader" means you read deeply within the genre and form in which you want to write. You also read widely, across genres, to learn what you can from them.
As you're reading, you're appreciating, yes (or bemoaning, as the case may be), but you're also analyzing for form and technique. Why does this first chapter work so well? How does this character's voice differ from that one, and why? How is this novel (or short story or essay) structured? Why is this scene so compelling or so flat?
I've been part of a writers' group for 20 years. I've also edited for hire, taught college composition classes, ghostwritten, and reviewed hundreds of submissions for Bailiwick Press. So I've done oodles of critiquing as well as "here, give-me-that" deep revision/rewriting of others' drafts.
New writers almost always stink at form and technique. Of course they do! How did the first meal you ever cooked turn out? What about your first vegetable garden or your first weld or your first watercolor?
Make no mistake: Writing is craft. It takes years of practice (I more or less ascribe to Malcolm Gladwell's 10,000-hour rule) as well as study of the medium, which is words strung together in an effective way, a.k.a. good writing.
When I do school visits, I tell kids there are two secrets to becoming an author. The first secret is to practice, just as you would need to do to master the cello or become a soccer phenom. The second secret is reading. Lots and lots of reading.
The thing I don't tell them, because they don't need to know this until they're older, is that eventually they'll need to learn to read as a writer, with a curiosity about and passion for form and technique.
I've seen many pre-published or published-a-little-but-often-rejected writers get stuck because they don't do this. Their craft doesn't improve beyond a certain point because they never learn to (stoop to? take the time to? think they have to?) hold up their work against the best writing in their genre—not to tell which is "better," but to understand how—sentence by sentence, page by page, chapter by chapter—they compare in form and technique.
I'm not talking about reading for pleasure, because this kind of analytical reading-for-writers does take some of the fun away. But I think that learning to pull aside the curtain on good writing is also a kick. Because when you look really closely, the best writing will reveal its secrets to you. Learning how to apply those secrets to your own work...that's another matter, one that counts toward your 10,000 hours.