The occasional typo wouldn't bother me--I know I have them too, although I proofread every post (there are times I've written, proofed, published, proofed...and didn't catch the typo until I got my post via email). But several typos in a post, in many posts...I'll find something else to read. Same for bad grammar or text-speak; trying to get the sense of a blog post that uses "u" and "lol" and "evry1" all the time would give me a headache.
The way I view my blog is that it is a public representation of who I am, and it's important to me to have enough pride to show myself well.
Typos don't bother me as much as obvious spelling/grammar errors. Sometimes I forget to run the spell checker before I hit publish so I must be forgiving of others. I know my thoughts run faster than my fingers can type so I often make typos, I just usually fix them before I hit send.
As for making me not go back to a blog, it would have to be pretty bad.
Ooh I hate text speak. Even when using my Windows Live messenger I don't use chat speak except maybe lol and ttyl. I don't use u or 2 luv or anything else and I certainly don't want to see it in a blog post. LOL is about the only thing I would be okay with in a blog post.