If you use email in your marketing (and you probably do) it's worth having a look at Return Path's holiday email consumer survey, which you can download as a PDF here. There's lots of interesting stuff there, but one point in particular that I think bears repeating:
Almost half (44%) of respondents say they receive high volumes of "junk" from marketers - defined as "email from companies I know that that is just not interesting to me."
It's not too hard to set up spam filters, and many of the consumer email services have good ones. But if you're sending junk (vs spam) you're not doing yourself any favors; customers notice. And they take action - according the survey, half of them go to the trouble of setting up filters to delete such mail. And many also report junk marketing messages to their ISPs as spam - which can cause the sender a lot of trouble. If customers do that enough, your mail is likely to start getting caught in spam filters.
Too often, email marketers have set the bar too low. If someone has opted into a mailing list, we think anything's okay. The problem is that the most important spam filter is the one in the recipient's head. If you send things that seem like junk to them, it doesn't matter that you had a good opt-in process - your messages are going into the trash, and quite possibly damaging your sender reputation along the way.
There's lots more in the survey - download it and have a look.