Punctuation: capital letters, full stops in the right places, paragraphs!
Grammar: word order, verb forms, adjectives/adverbs, past perfect and future
Vocabulary: prepositions, avoid very/really (if possible), use fancy words
Flow: avoid unnecessary repetitions (don't start sentences with the same words!), variety in lenth of sentences, try to keep it interesting for your reader
Spelling: were vs where, there vs their vs they're, -ful vs full, -s vs 's