watching this video about yahoo's past, present, and future, i picked up on an idea of Terry Semel's regarding contrasting Cable television and the Internet.
when HBO first started, they had to purchase all their content and distribute it for a subscriber's fee. eventually they got some content "exclusively" ("see 'Splash' only on HBO!"), and more recently they have developed their own premium content (Sopranos, Sex and the City, etc).
The idea is that at first, companies like yahoo simply delivered content (search engine, advertisements, etc), with a little bit of premium content (yahoo games, fantasy sports teams, etc). their goal was to get more people to become Yahoo users.
currently people use yahoo because it is not only an easy way to get to the third party content they desire, but because Yahoo is the only one with a certain piece of content, or the cheapest place to get it. eventually it will be that you use Yahoo for something that is totally Yahoo-specific. (yahoo mail is not yahoo specific, i can get free webmail at several places, hotjobs is not yahoo specific, etc). developing this product that will draw users simply through its existence solely on your site is the trick.
that's the next killer app.
just think of how many people subscribe to 24-hour HBO just to watch the Sopranos once a week. thats where you make money.