E-mails are the most common way for people to communicate nowadays, and it offers a way to reply when you have time. You can get instant feedback, attach files etc to give a person all necessary info. On top of that, you can easily see when e-mails arrived or were sent, and make a decision for your actions based on that.
So, something that really annoys me is when I don't get a reply at all to e-mails I send. I understand that people are usually extremely busy, and I definitely don't expect a reply within five minutes. But I think that people should really be able to reply within a week or so; not doing so is disrespectful, and bad business practice if that's the context. Some of the big names, be it a company or a famous blogger, are pretty good at replying, but some are really terrible.
My worst example is a job I applied to a couple of years ago. Five months later I got my first reply...
Some people get a lot of e-mails, and I sympathize with that. But I still think that a week is a long amount of time to produce something back. Not every reply has to be a novel; it can be a sentence or two, acknowledging that they got the e-mail and answering the question or promising to get back. For the ones that don't think that they don't have time for this, set up a polite auto-reply explaining the situation and if/when you will get back. Yes, this will let spammers know that the e-mail address is valid, but still, this mostly applies to large organizations and well-known names where the e-mail address is publicly available anyway.
People keep arguing about top-posting, if one should send HTML email and so on. I don't care. Just give me and others a reply and most of us are fine.