Word choice as an author is what makes you an author, and correct spelling and word placement are paramount to conveying information the way you envision it when you write. This is where most people focus and for good reason. Spelling and grammar are the foundation for writing. Works littered with misused words, awkward punctuation, misspellings, and confusing syntax can immediately put off a reader.
There are so many free spelling and grammar checkers out there that most of these issues should be gone before someone reads a book, but they are not fool-proof. So here are some things that sneak by all the time and writers/reviewers should keep in mind.
Spelling & Word Usage:
-Wrong or confusing words used? (inaccurate connotation, break vs brake, etc)
-Does it seem like the author pulled up a thesaurus and chose a word that doesn't quite fit?
Grammar:
-Are quotations and punctuation used correctly? If not, is the implemented format at least stylistic for the characters, plot, or tone?
-Is the tense consistent?
-The sentence may be "grammatically correct" but does it make sense?
Vocabulary:
Do words you don't know, at least have some contextual definition or understanding?
Are the same words used two or three times in same paragraph, or even the same sentence? ("yelled" used twice in a sentence, instead of using "shouted" one of the times)
Having a large vocabulary is needed as an author, but are there countless words that make the meaning of sentences and paragraphs convoluted?