The functionality that caught my eye was SuperPreview option. It allows developers to render the pages your working on using chosen browser engine within your development area. Available are all popular versions of IE and any other browser installed on your machine. It actually uses the real IE6 engine instead of any emulators. It also allows you to display the same page on multiple layers, each rendered using different engine. This allows you to find the differences much quicker.
You can read about this cool option in this article.
There is also a separate version of SuperPreview which doesn't require Expression Web but it only supports IE. You can download it from here. Here is how my blog looks like when IE6 & IE8 overlay:
PS. I just wish that SuperPreview functionality was available in VS2010. The Expression Web seems as a new product only from marketing perspective. Its designer/developer area offers exactly the same options as in VS (code/design/split views), except the SuperPreview of course. Since VS already supports ASP.NET development, it would be logical to simply add PHP support to it instead of releasing a new product. When I asked tech guys from Microsoft they couldn't really explain this, which I think proves they think the same :)