This ties in with a question on Quora on why Dropbox is so successful. http://www.quora.com/Dropbox/Why-is-Dropbox-more-popular-tha...
Knowing where to draw the line between a basic value proposition and extended features is crucial to success. I have this same dilemma. Users have been requesting features to one of my websites, but I am unsure if adding these features won't detract from the simplicity of the experience. Currently users do 10 pages a visit, which to me, tells me they understand the flow of using the website.
It is an exciting trade-off!