I probably become annoyed and frustrated too easily by small things but professors not replying to somewhat urgent e-mails really gets me. Granted, school is not in session yet but I know the professor is in his office, all I need is a short little e-mail answering a few questions. How long does that take? 5 minutes, maybe, if he types slowly.
I try to reply to e-mails as soon as possible or if there are a lot, set aside some time to reply. It is just the courteous thing to do.