Well there has been quite alot of work that went into their URL Rewriting, because I am using BE 2.5 i can take advantage of one instance with many sub-instances. However i wanted to reverse the virtual path so instead of http://domain.com/path i reversed
it to http://path.domain.com/ as virtual paths. This may sound simple but I tell you took alot of work to locate and update everyplace in the source code.
Now auto blog creation was a little difficult as I need to remove the API's that are in the system that could have security breaches. So the blog creation is my own brew with uses the back of BE and a bunch of new extension methods written in to create the
blogs automatically. Additionally I had to write a new front end that does the user registration, creates a blog, create the users and roles, sets all the settings to custom settings this was alot of work to get correct.
The dashboard was pretty easy but still took some time, I had to remove some fields based on blog role (ie Primary or not) but maintained the integrity of the ajax js scripts.
Alot of work now is addding settings and features such as. Blog Logo's, Twitter & Facebook oAuth helpers, working with the theme's to remove generic items and replace them with user bound settings.
What I would like to do next is create a Theme builder which would allow people to select colors, fonts, background, heights\widths etc like blogger.com.
Alot to do but alot has been done!