The Acrobat 9 release offers lots of great features, but in reality, there's a little preference that just may be more important than any new single feature itself.
In the past, I've posted here, here, and here about how the overprint preview mode is so important.
But as many people are quick to point out, it's one thing if you know (or remember) to turn overprint preview on. It's quite another thing to get your clients to turn overprint preview on.
And that's why Acrobat 9 is going to make our lives oh-so-much easier.
Acrobat 9 now has a preference (which is on by default), that automatically senses whether a document contains overprints in it. If the PDF DOES contain overprints, Acrobat 9 automatically turns on overprint preview, so that the document displays correctly. Yay Adobe! This feature will be available in the free Acrobat Reader application as well, so you'll just want to make sure that your client downloads the latest version of the Reader (version 9 hasn't been posted yet, but I expect we'll see it in the several days or so). Naturally, getting your client to update their reader is yet another challenge :)
I believe the default setting that Acrobat 9 and Adobe Reader 9 will use is for PDF/X-1a files (which suffer from this problem the most), and there's also an Automatic setting that will work on any PDF document.
On a side note, this is the kind of stuff I like to see from Adobe. Applications that are smart enough to make changes as needed (where appropriate) is something I'd like to see a lot more of.