Unsubscribe rates are affected by many factors from the very obvious (covered in more detail below) to the extremely sublime (such as the recipient having too many emails today, to having a hangover and being in a bad mood, to not liking the layout or color scheme of your message, to mis-reading the subject line and thinking it was something else, to who knows what else).
The main and constant factors contributing to Unsubscribes - and the ones you can control - are these:
Permission based messages - If you obtained permission to email someone more than six months ago but haven't sent them anything during that time, they may have forgotten they've given you permission, or their circumstances may have changed so that your messages are no longer relevant to them. Or maybe you have never gotten permission to email (by this we mean you have communicated directly with each individual by mouth, posted letter, email message, sign language, smoke signal, carrier pigeon, subscription list or link on your web site, or some other mode of communication, "Hey, I'm going to send you some emails!" and they said "Ok, Great!"). In either case,we strongly suggest you get their permission or remind them that they gave you permission. (See our page on General eMarketing Tips & Etiquette - specifically the Frequency section which provides instructions for "warm-up" or "reminder" campaigns).
Frequency of communications - If your recipients feel you're sending too much too often, they are much more likely to "opt out", or to report your messages as Spam. In most surveys, this is the #1 reason for unsubscribes, people! If you narrow your target audience for each message, chances are, you'll email each contact in your data list less frequently. Which leads to ....
Message content - if your recipients perceive your content as inappropriate, not applicable or redundant, they may unsubscribe. Again, the "targeted message" approach with email has been proven time and again to work WAY better than the "spray and pray" method (you know, where you send every message to everyone regardless of its applicability, and pray for high response and low unsubscribe rates).
Incorrect Merge Tag data - if you're using Merge Tags to auto-insert contact data from your database (such as recipient name, address, etc.,) and you haven't kept your data up-to-date, chances are you've got outdated data, dude. It's a HUGE turnoff when you open an email message that's got bad, incorrect or old data about you, true? It is to your contacts, too. Check out our page on Data Management for good list hygiene techniques.
Message load time - messages that contain lots of embedded images, Flash or other code that must download to be viewed are just baaad juju! They BEG recipients to unsubscribe. Depending on the recipient's firewall or security settings, they may receive "warnings" about message content that could alarm them. Or they could simply get frustrated if they have to wait for things to download before they can view or read your message. Links to hosted images or Flash content are a great way to provide additional content (and interactivity) for your messages without adding extra load time.