What makes Twitter interesting is just how easy it is to use.
Here's my rough timeline of web ease-of-use.
In 1989 comes the web. Putting content on the early web is hard. Getting a server, learning apache, hand-rolling HTML, maintaining consistent headers and footers and navigation. Many steps, difficult to execute manually.
In 1997 comes blogs. Free (or nearly free) web-based content management systems. Suddenly, all the tedious HTML and CSS details vanish. Web publishing becomes easy. Web publishing explodes.
In 2006 comes Twitter. Type a sentence, and blam -- it is online. Reach the entire world, instantly, for free. Zero complexity.
Twitter's rapid growth, I think, owes much to its blinding simplicity and speed.
Taking an online order will never be as simple as Twitter's 140 characters. Bill-to, ship-to, gift options, multiple payment types, shipping options -- e-commerce is a complex beast.
There's a lesson for online retail in Twitter's growth: people prefer easy-to-use fast web apps. And so use them more.