I also wondered why the Twitter team didn’t include more features, and why they left it up to external services to do things like search (which they eventually acquired by buying Summize). But the more I thought about it, the more I realized that the smallness and lack of features is actually a positive, not a negative. What Twitter did was strip all the clutter of many social networks away and pare things down to their essence.
A tweet is like the smallest possible building block of online interaction — the atom of social media (an idea I wish I could claim was mine, but appears to have occurred to others as well). With those atoms, plenty of other services have sprung up to build larger structures.
(read the rest of this post at GigaOm)