So I've written a fair number of reviews on this blog. I don't know the exact number, but I think it's more than 10. And frequently, I know that I point out some grammatical errors, or overuse of adverbs, or some such, and say that the book needed editing.
I think in the rush to put out books, publishers are skimping on the editing. (Or sometimes, authors get too big for their britches...hint, hint J.K. Rowling...and refuse editorial help. There's one scene in Harry Potter And The Order Of The Phoenix, right before Harry goes to his trial at the Ministry of Magic, where there are just too many characters saying things ____ly.) Sometimes people legitimately don't know, that they don't know something. Like how to write a proper sentence in English. (I know that last one was incomplete. This is a blog, not a book.)
Some people say that writers are not editors. It's not necessarily an easy skill. But if you like to write, and think you want to be a writer, then surely you do a lot of reading. I was going to say that this ought to teach you about proper grammar, but then I realized as I was typing this how many errors I have seen in books lately. Reading books -- especially fantasy books these days -- teaches you the use of large numbers of adverbs, improper use of "I" and "me," etc.
You want people to focus on your story, your characters, the world you've created. You don't want them fixated on the individual words on the page. Good grammar and language really shouldn't be noticed. It should be taken for granted. It is VERY easy to spot bad grammar, overly simple, or needlessly complex language. I complain about it all the time on here. It is much less common to read something and sit back and think, wow, that was really excellent writing and use of language. (Though I do this with Guy Gavriel Kay's work all the time. Reading Kay makes me a better writer, I've found. As I'm typing up my manuscript, I can guess at which points during the writing I was reading him and at which points I was reading someone else. Just wish he wouldn't write some scenes in present tense.)
Maybe people are spoiled by e-mails, text messages, Twitter, and Facebook. (I don't use the last three of these.) If everything you read is on the web, then your expectations might be lower. I'm not claiming this blog is going to earn me any writing prizes. That's not the point. It is useful to have discussions about the fantasy genre, I think, and these are naturally informal. But there really should be higher expectations when it comes to a printed novel.
Bottom line: Have someone read your manuscript. Preferably several people. You don't want people like me picking apart your grammar online for all to see.