Remember, there's unlimited reading material to choose from; your not-worth-reading detector should be sensitive, because false negatives cost much more than false positives. When reading an author for the first time, unless I have a strong recommendation or other quality signal, I will stop if the first incidence of stupidity precedes the first insight, or if there are no good insights in the first 500 words or so.

For superficial signals like spelling and overuse of emphasis, I divide them into two categories: things a good writer would do if they were rushed, and things a good writer wouldn't ever do. Typos, missing words, few citations? You're looking at an unedited draft; whether that's okay or not depends on the context. Bold italic all-caps large font? Crackpot.