The huge response Posterous gets from their fans for every micro feature they release is because they already have a fan base. The more community or users you have who depend on your service, the more they want a feature. If you start off with an empty blog platform with every feature wordpress has, nobody really cares. OR, if you start off with one really distinguishing feature (Post by Email) and you instantly gain a huge user base who like that, they wait in anticipation for all the small features you can find elsewhere.

Posterous has an awesome foundation to build off of, and because of that they have a real advantage. They can get feedback from a lot of users, including requests for features. What to build? That is easily solved just by listening to what people have to say. They can validate features to build, and are justified in their time by the constant requests for these features.

As soon as they release a feature, they know it's going to be big since the community has been asking. And when it's there, the community feels that they were just given what they've been wanting. Nobody blames them for not having it in the first place, knowing this is a growing app, and are glad to be early adopters.

These guys (garry and sachin) are in a great position, and it's nearly impossible for them to NOT build a great product with all the feedback they are getting.