It can password protect it, and you can link things from page to page. Some of the best sites don't do anything "impressive", really...

For example?

Reddit, LiveJournal, FaceBook?

Wow. I can't remember ever seeing a negative nostrademons comment. Obviously that's not earth shattering but it is feeling like downvoting is more generally on the rise around here.

You can criticise many aspects of Facebook... but it does a lot of impressive things.

Actually? I sort of think that facebook's impressiveness is in its ability to scale the features so nicely.

I don't think there are any features on facebook that couldn't be made by your average ycombinator news reader (they might be slow as all hell, but I bet most of us could create working demos of most features).

Facebook does a lot of almost invisible but extremely smart things. Two examples: when you specify the type of relationship when adding a friend, Facebook uses the info to decide how much information to present about the person in your news feed. Or when two people list themselves as being in a relationship, Facebook will show the most recent picture of the two together (if such a picture exists, and privacy permissions permit it in your case).

Nothing fancy. Just extremely well thought out.