If you are interested in building a site that is similar in behavior to say WordPress.com then you need to be [or have at your disposal] a PHP developer that has a good working knowledge of Word Press and the architecture of Word Press MU.
The key to the "community" concept that Word Press.com puts forward is primarily the role of the hosting blog. This blog is not entirely 'themed' in the same way you would theme a user blog. This is because the hosting blog deals with not only the articles/categories and pages of the host itself but it gathers, collects and offers up information that comes from within the community it represents.
To emulate this behavior you need addition PHP functions, database tables and some well-written mySQL queries that are scalable and reach across the entire site. These are used to collect and display the information that extends the role of the hosting blog.
Free PHP code for this purpose has not been heavily developed nor is it readily available. There are very few useful functions that come with WPMU that you can use to build on. The signup, login functionality, a few functions dealing with blog activity, etc is about all there is.
Global categories, global tag clouds and anything else that would be necessary to create "site-wide" exposure would need to be created from existing plugins or developed yourself. There are a few plugins available for download from http://wpmudevorg.wordpress.com/plugins.php but most that concern themselves with "site-wide" functionality are not really suited for large communities.
It is possible to emulate a WordPress.com or myspace style site in which the "community" atmosphere is obvious but you won't find the pieces you need to build it available in the same easy way you find plugins available for WordPress.
Having said all of this, if you're just interested in creating a multi user blog that doesn't represent a community (other than the "Get yourself a free blog" on the home page) - then you have everything you need in home.php