Design or accident: How did Tumblr become Tumblr?

When Tumblr first came out, it was promoted as "instant, no-overhead".   The post on the Union Square Ventures blog about their first investment in the company is all about personal expression, aggregation, curation.  When I first talked to David about it, he stressed the simplicity.

You know what no one seemed to be talking about?  The community. 

A while back, I recommended Tumblr as a dead-simple blogging platform to a friend.  Now, if you're just writing full blog posts and not taking part in the Tumblr community, you'd be a bit of an odd duck.  It would be the 2009 version of someone who decided to write an economics blog on LiveJournal. 

Hmm... LiveJournal.  Is Tumblr the new LiveJournal... or LiveJournal for adults? 

In both cases, there's clearly something *more* going on than just publishing.  In Tumblr, there's a community with it's own social currency--reblogging.  Nearly three quarters of my dashboard is reblogs of others.  Do my friends only express themselves in relation to others--unable to convey original thought?  Or is it a function of how easy this is to do?  Was this community behavior evolution or intelligent design?

The other aspect of services like Tumblr, Vimeo, or Foursquare, for example, that is tough to replicate is the nature of the initial users.  Doostang has this attribute as well--although instead of hipster geeks, it started out with private equity geeks.  That has not only dictated the makeup of the Doostang community, but has ultimately impacted the business model there as well--the site is now a pay for placement recruiting service. 

So, what is Tumblr?  Is it the application?  The people?  An emergent behavior?  Can it be replicated?