Features vs. innovative technology. Which one leads to the development of a successful web app? Some may see feature set and innovative technology but I think their is a distinction that is pretty simple to follow. For instance, Google has always built it's applications based on innovative technology.
Google Search is light on feature set, in fact, it started with just one - search, but through the usage of their innovative technology, they've made a winner. On the feature set spectrum, the easiest examples to use are the social networks. MySpace became viral largely because it offered a variety of features, none of which were incredibly technical or innovative.
LinkedIn is another example of such a company. LinkedIn has been growing in popularity largely due to their feature set. Again, they are not reinventing the wheel here, just releasing enough functionality to keep people interested and modeling them in an interesting and addictive way.
For instance, when they launched their answers functionality, nothing about it was fundamentally difficult. As a user, you can ask a question, and get answers from the community. You can then pick the best response of the bunch. The reason it caught on was that they chose to feature it prominently on a users profile. In fact, it's the first thing you see when you log in.
Spreading the questions through the network by making those that pick up steam easy to click into, and rewarding the users that answer with the most accurate response was a great way to add stickiness.
It seems as a web app you typically need a good combination of innovative technology and addicting feature set. The successful companies of web 2.0 have all been able to master this.