Microsoft is taking browsers seriously again and this is both good and bad from our point of view. Yes, it means improved features but new features generally mean incompatibilities and code re-writes. The first important news is that IE9 will very probably not run under Windows XP making it another step in marginalising this older operating system in favour of Windows 7.
The key features of IE9 from a programmer's point of view is its almost agressive support for HTML 5 - "it's HTML 5 through and through" - and CSS 3. It makes sense not to stress too much the past history of patchy support for earlier HTML standards. The browser will honour doctype tags but you will be able to force it to load pages in the style of earlier browsers.
Microsoft also finally adopts SVG and this almost certainly means the death of its own VML. Native support for SVG in IE9 means that we can at last start developing vector graphic applications for the future without having to worry about plug-ins and issues of support. On Windows machines SVG is accelarated by use of DirectX 2D.
The same developer tools that have proved so useful in IE8 are included in IE9 but with significant performance improvements. New is the network tool that allows you to capture and example HTTP/HTTPS network trafic.
You can try out IE9 at a test drive site and download a platform preview from the same site (no registration currently required) and Microsoft claims to be keen to recieve developer feedback.