There are many different (possible) reasons why an e-mail sent to me goes unanswered for a very long time or never gets answered at all. Here's a quick rundown:
Sometimes, things go wrong in cyberspace. A server may have conked out and lost your message while en route to me. The virus gate at work may have malfunctioned and deleted your message by mistake. The bottom line is, the internet is not 100% reliable when it comes to e-mail delivery.
I don't do flames. I can pretty much tolerate disputatious messages from people. As long as there is provocative intelligence displayed, I will always respond in one way or another. But if you start calling me names and using racial slurs, you're instantly put on the blocked senders list and it's over. Life is too short to spend sparring with difficult and close-minded people.
If you're not in it for seriousness, I'm outta here. In this fast-paced world, time is of the essence, and I make it a point (before responding to a message) to do a quick once-over and decide whether it's worth my time to sit down and respond. If I see that it's coming from blowmybig@dong.com, I won't even bother.
Down and out laziness. Nobody's immune from slacking. I do get afflicted with it from time to time, usually when I'm too tired, stressed out about things or physically ill. But I always make it a point to respond. It may take me a while, but an answer will be forthcoming. I believe it's only fair and respectful to write back to people (excepting flamers) who took the time to write you. You simply have no excuse to be impolite to people, especially if they say nice things about you and your site.
It's too much, it's too much! Since email is so easily posted and read, the sheer volume of it can sometimes be a bit overwhelming. Factor in being human and you will inevitably have things falling by the wayside. So, if you've sent me something a while back and I haven't responded to it, a little reminder would be much appreciated. That is, of course, if you're not too busy being upset with me and sticking pins in a voodoo doll bearing my likeness. ;-)
I hope that clears everything up.