I would add to the spelling list (thanks, WordPress):

• Knowing how to distinguish between its/it’s, your/you’re, there/they’re/their . . . and
• Knowing how to use nominative and objective pronouns (nominative = pronouns that name; objective = pronouns that receive action)

These are musts for any polished piece of writing.

What are your spelling or grammar pet peeves? Share your frustrations here!