Yahoo has been plugged with service shutdowns, search engine inadequacies and numerous other problems. Yahoo is still 2nd rank, behind Google. Yahoo and MSN have tried to penetrate the ad market, but the real reason why they have not been successful is because they lack a traffic metric. They do not have their own system of ranking traffic, for search engine purposes, or for site ranking purposes.
Yahoo ads have been an issue from the beginning. They do not have as much prestige or power as Google. MSN might as well shut their online advertising division down because they have not made much of a impact on the web. It is going to take a miracle for any company to get over Google. However, if they are innovative or creative enough and have some great management I think it is possible to overthrow Google. Apple seems like a good contender for this position but I doubt they are interested.
Google is beginning to become stale. Their creative process does not seem as fast paced as it has in the previous years. They still rank very high and have some very unique new projects in the pipeline, but it just seems like they are lackluster. They need some new creativity!