Yahoo forces advertising onto the viewer. Their main page always features an ad, whereas many search engines do not: Google, AltaVista, Dogpile, Live, etc...
Another example of rather harsh Yahoo advertising is their Geocities sites. Geocities allows users free webspace, but only so long as they comply with tiresome policies and are willing to put up with Yahoo's advertisements on the right hand site of the page. Google is a large site which has taken a different road: allowing the users to control advertisements and take part in the profit. A perfect example is this very site: Blogger/Blogspot, which allows users to somewhat modify html and use Google's Adsense.
Yahoo's very uncreative way for profit will be its downfall.