We ran into this, too, and have been dealing with it on some level for the past 6 months. It's probably not the content of your mail; this year, many major providers have gotten very aggressive with their anti-spam tactics, especially Microsoft.

1) You'll probably need to get a dedicated IP for sending mail if you're going to send at any volume; pretty much all the major operators calculate some sort of "score" based on your IP's record of delivery. The "score" includes: how many mails you send, how many are to "real" recipients (not deactivated users/nonexistent accounts), how many people complain about your mail, whether you not you hit any spam "trap" accounts, etc.

2) SwellJoe is right about a reverse DNS record that points to your mail server; it's basically required these days. A valid SPF record (http://openspf.org) is also basically required. You can also use DomainKeys or DKIM (http://dkim.org/) to improve your deliverability to Yahoo! accounts. If you're sending invites or other user-generated mail, read this in particular: http://www.openspf.org/Best_Practices/Webgenerated

3) Process your bounces. If you keep sending mail to a deactivated account, this looks very bad. Microsoft in particular has been super-aggressive about this. Set up a script that listens to incoming mail to your postmaster account.

4) Enroll in Microsoft's JMRP and SNDS programs (http://postmaster.msn.com/) -- there's a bunch of other stuff about Microsoft's practices there, too. SNDS lets you know how your mail is treated by the Hotmail servers, but you need to send at least 100 mails on any given day to get the data. JMRP forwards complaints from Hotmail users to you so that you can unsubscribe them.

5) This is the only guaranteed way to get into Hotmail --If you're serious about your site and you can cough up $400 to apply (and $1000 a year), Microsoft maintains a whitelist through a company called ReturnPath (http://returnpath.net) and a program called Sender Score Certified, which has some pretty intense requirements. The process can take a really long time, but we just went through it, and it's not too bad once you get the process going. (http://www.senderscorecertified.com/)

And finally, a bit of speculation -- I was doing some testing a few months back, and it seemed to help to have both an unsubscribe link and a postal address at the bottom of the mail, particularly for Yahoo!. Not sure if this is still true, but it might have something to do with CAN-SPAM filtering.

Hope that helps.