Saying, "Right?!?" at the end of every sentence. I don't know if it's more grating than the incessant "you know" that it apparently replaced or not. It's a close call...right?!?
Using the reflexive unnecessarily and inappropriately. As in Gray Tide's post #10 above.
Using "I" in the objective case. As in, "John went to the beach with Jane and I." "I" is a nominative noun. John and I went to the beach with Jane. But John went to the beach with Jane and ME. [dammit]
Not knowing the difference between, "your" and, "you're."
Saying, "If I would have known..." It's, "If I had known...." For the record, "If I'd have known," is just as bad. It's a contraction of the mangled phrasing.