One thing you have to be prepared to do if you move from Joomla or any PHP portal to DNN is get used to paying out money for stuff. Once you get over that bit then there are tools around that will do most of what you want. The DNN developer crowd is however definitely slower off the mark then the PHP scene in terms of implementing the latest web fads and you'll likely find yourself relying on what 3-4 solo developers do with their weekends. DNN itself serves primarly as the user and template management aspect of this and you need to find third party modules that integrate to provide the actual functionality you want.

Have a look at modules from DNNFusion, Smart-Thinker, Bizmodules and Onyaktech. However, each variant on the Myspace theme has specific caveats and you need to be very specific about how you want yours to work. Also keep an eye on Snowcovered.com and the DNN Marketplace for modules.

Always keep an eye on the PHP scene as well.

Regards
Rob