Email providers, from big public services (Gmail, Hotmail, Yahoo!, etc), to corporate solutions and spam filters (MessageLabs, Postini, etc), constantly monitor your sending activity. The more they trust you and believe you to be a good sender, the faster your emails can be delivered and more likely they are to place well in their users' inboxes (and avoid their junk folders). Conversely, if they believe you to be a poor sender then you emails will be sent more slowly due to deferrals.
There are two main factors that influence your sender reputation: bounces and contact engagement.
If you send to a list that yields a lot of bounces, emails sent to non-bouncing address may suffer too. Email providers will identify your sending as spam-like behaviour; to them it closely resembles how spammers operate. For example, spammers might write a script to guess email addresses @yahoo.com. To an email provider, a sender hitting lots of non-existent email addresses looks very suspicious.
If an email providers sees that many of your contacts are opening and clicking your emails, then there is a much higher chance that you email will be accepted quickly and avoid spam folders. Conversely, if contacts are not opening and clicking, and are perhaps even flagging it as spam, then you email delivery will be slowed down or potentially even blocked.