When it comes to new services, simplicity is one of the major keys to success. People switched to Google because it was easier - it shaped itself around the search bar; not banner ads, media content, or hidden sponsored URL links. Part of the success that 37Signals has is a direct result of their service being so simple. What would their initial growth look like if they came out with all of the features they have now? Most people wouldn't have known how to react to all the tools - it would have overwhelmed them.
It doesn't matter what your business is - keep it simple, stupid. Whether you're a search engine, an enterprise software startup, or someone focusing on a niche market - keep it simple. The faster your users can get in, do what they need, and leave - the better. In fact, I think I'm going to make speed our number one priority. If a user can't do X action in Y amount of time, something needs to change.
Another killer of simplicity is feature creep... That is, when a service adds too many features too quickly, complicating the user interface AND development. We're picking an easy solution for that problem - we're only going to focus on the core niche and create a site with one feature. Think of eBay - they started with the simple auction listing - nothing more. More advanced features can be integrated if the service gains popularity - otherwise, who needs advanced features without users?