When Microsoft launched the 2007 version of Outlook it used Word to render the emails. This meant that certain features used in broadcast emails in the past were no longer available. Most designers have found new ways to layout email templates to accommodate this but we still occasionally get a design that causes problems.

In short Outlook 2007 is no longer able to use/show the following in emails.

Forms
Not commonly used as the aim of the broadcast is generally to get visitors back to the site where you can interact with them more fully.

Background images
Still common in a design that comes in to us but getting better, the only way to adapt this is usually to have a solid area behind any text and the place the image on the page outside the text area. Some email clients still don't show this off well but it gets the job done.

Animated GIFs
Outlook 2007 will just show the first frame of any animated GIF and again that can be linked through to the website where the full animation can be used in an article.

Float or position commands
Again there are ways of placing items on a page that can get around this but the challenge is now for the designer to find layouts that still work well.

Bear in mind there are still some email clients that are considerably more restricted than Outlook. This is Microsoft's explanation

This article explained the effects  in a forum

Alt tags in images
Disappointing this as it is a routine to add Alt tags to all images on the website to help people using the accessibility features.

Flash
Again this can be replaced with a fixed image and a click through. People were starting to try and use Flash in emails when 2007 was launched but the restrictions seemed to have slowed that down.