Pinker's Style

I recently read The Sense of Style: the Thinking Person's Guide to Writing in the 21st Century! by Steven Pinker. Pinker covers everything that confuses and confounds writers. :) For example, we all know a pronoun is a word that take the place of a noun. In fact, there are different kinds of pronouns:

• A nominative generally is the subject, or actor, of the verb. Nominative pronouns are I, he, she, we, they, and who.
• An accusative generally is the object, or receiver, of the verb. Accusative pronouns are me, him, her, us, them, and whom.
• Genitive is primarily a way to indicate possession; it's a noun that modifies another noun. Genitive case is marked on pronouns: my, your, his, her, our, their, whose, its and on noun phrases with 's.

I sometimes have trouble with I verusus me. Using this information, a writer should write: I was down by the schoolyard. Or, even: Julio and I were down by the schoolyard. But a writer can write: Me and Julio were down by the schoolyard. One of Pinker's messages is language evolves and writers should use their knowledge and experience to write what works for a particular piece.

Another tricky one for me is who versus whom. But the nominative/accusative difference should pin it down. Who kissed the bride? Whom did Henry kiss?

What grammar issues do you find tricky?