Sitefinity 5.0 is really Sitefinity 4.5, just rebranded. I am getting lots of calls from customers in a panic that this will be like going from 3.x to 4.0. It won't be like that.
Sitefinity 5.0 has some nice new features that people have been waiting for: http://www.sitefinity.com/web-content-management/roadmap.aspx
Notably, the Module Builder will have an import/export feature, thus making this feature ready from prime time.
It's interesting (and disappointing) that the Multi-device page designs are NOT going to be available in Sitefinity Standard! Everyone needs to have a mobile version of their website and I hope they put something together for the Standard package.
Sitefinity Thunder is also a welcome feature. It's interesting that Sitefinity has decided to add all of this functionality through the use of Visual Studio. Granted, we were using it anyway, but with the Module Builder and Widget Editor already within Sitefinity I would have thought that more functionality would be moved from the development environment to the browser.
Last but not least is the Forums Module. This was the most surprising feature left out of Sitefinity 4.0, considering it was included in Sitefinity 3.x. We get a lot of requests for Forums, so I look forward to this release.