Look at Tumblr's landing page:

That’s it. Sign up is dead simple.

Can you imagine the conversion rates they get out of that page?

Moreover, they have one of the best taglines of any service: the easiest way to blog.

What else is there to explain? They don’t brag about features like “like”, “reblog” or (ugh) “tumblarity.” Tumblr is the easiest way to blog. Anyone, your mother included, can understand that.

Great design is not how it looks; it's how it works for the end user.