One one end of the spectrum with WordPress multisite might be a site that’s locked down. It has a narrowly defined purpose and very limited themes and plugins. You may find there is only one theme and the plugins are all network activated. Your choice in this space is still greater than what you’d get in Facebook or Twitter but it’s pretty tightly controlled. This can be good for some people and some purposes. It’s consistent. Very little can go wrong because there are so few choices. People are not overwhelmed by choice.
On the other end of the spectrum is what we tried to do with Ram Pages. You have a couple hundred themes and a couple hundred plugins. The goals is to make pretty much anything people wanted to do happen in that space. You can’t install your own themes/plugins but we’ve covered most of the bases and would respond to unmet needs pretty aggressively. If we didn’t find a theme, plugin, or combination that met people’s needs then we’d make one.