I have a system that works well. It has built-in affiliate support for Amazon and ebay (UK and USA). It has integrated google ads. It has product pages for links to affiliate sites. It has mail list registration. It has forums. It has a links page. It has an article list and detail pages. It has news pages. It has support for analytics. It has RSS feeds for everything. It pings the RSS aggregators when things get updated, to ensure indexing. It can also access a database full of affiliate feed data to have more than amazon products listed, but I'm not using that at the moment.
What it doesn't have is user facing admin screens, so it's good for me, but not really anyone else. I can't spend the time on making it user-facing currently.
I'm currently running 292 sites on it and each new site is one database row. Takes minutes to add a new site.
Forums are locked down to avoid spam (which is a HUGE amount!) so nothign publishes live until I manually tweak.
I have it all setup to use stylesheets to allow for different look and feel per site, but I haven't actually created any apart from the default yet, so might need a bit of work.
The sites pick up lots of long tail traffic (assuming they have some decent content/forum messages etc.)