The other day I talked specifically about the social features in the SharePoint 2013 preview. Lots of people have been blogging about SharePoint 2013, with many people sharing screenshots.
However, what you may not realise that its actually quite easy to try the SharePoint 2013 preview for yourself by signing up to try the Office 365 Enterprise Preview. After jumping through a few registration steps you'll have a preview site and more to play with.
The IT Professional Reviewer's Guide for SharePoint Server 2013 Preview (a glossy PDF) is also a good place to start if you want an overview of what's new, before diving into the more detailed online documentation found on Microsoft's TechNet site. Alternatively, there is good overview post about SharePoint 2013 on the Microsoft SharePoint Team Blog.
Its worth digging around the TechNet site, as you'll find some useful resources covering the features in the SharePoint 2013 preview - such as a one page overview of the mobile ecosystem that explains the mobile browser options and services (for example, did you know about the new geolocation field type that can be used in a list?).
Another area that I think a lot of the reviews I've seen are overlooking is the new app model in SharePoint 2013. This introduction gets a little technical in places, but see "Figure 2. Options for hosting apps for SharePoint" for quick overview of the implications of this new approach - i.e. you can host SharePoint on your own server, but run apps that are hosted in the cloud.
"Skydrive Pro in Office 2013 leverages SharePoint (either on premise or in the cloud) to provide secure and synchronized storage and sharing for your business documents."
As you can see, Microsoft has actually published a lot of material of its own and its worth going straight to the source to start learning more about SharePoint 2013 and related services.