Try to keep repetition in a paragraph down. Use the thesaurus. Even better, use the thesaurus function in your word processor. People love to read in the English language because they like the good feeling of hearing new and different words. Spellchecking is a must.
Every misspelled word makes many readers wince.
However, a blatant lack of spelling will turn the reader off. Grammar checking can be helpful. Be sure to disable the annoying options of your grammar checker. I mostly use it to catch little errors, and I have lots of options turned off. Feel free to invent words.