Google Search does its job if it sends you to the most relevant sites as fast as possible. Google OS tries to keep you using it to save and edit documents, photos, blog posts, events and more.
The services from Google OS might be search results, but not necessarily the best. While most people identify Google with Google Search, the company has to use its most valuable assets: the homepage and the search results pages to promote the rest of the products. That's why you'll see ads for Google Toolbar at the bottom of SERPs or promos for Google Talk, Google Checkout on the homepage. Google also uses AdWords to promote its services, but the ads can't stand out enough to be really effective.
The problem with Google's promos is that they're not context-sensitive, so users will find them out-of-place. The famous self-promoting tips tried to react to user's query, but they were poorly targeted and stirred a lot of controversy. The sacrosanct search results had an undisclosed ad on top.
So how should Google promote its services without affecting the credibility of search results? Gmail should integrate with more Google services and become a way to discover the usefulness of the rest of Google: read feeds, view your agenda, save notes, manage your attachments and the files saved in other Google services. Google Toolbar and Google Desktop should become more useful and bring the power of Google services anywhere you are. Google should secure other channels for promoting its products instead of relying on tips and promos.