There are major differences between Google and any other site that just happens to collect data because they need to:

One difference is size. For the average internet user, Google is everywhere and collects more of his data than any other company. Think Google Search, YouTube, Mail, Google Talk, Docs, Plus... it's everywhere. I wouldn't mind one company having access to my mail/contacts and another one having access to my cloud documents while yet a third one knows what I search for on the web... But the fact that all of this is linked and combined by only one company is irritating.

Another difference is that for every other site, I know that they "watch me" while I'm there.... and don't know me when I'm not. Nokia has a lot of my personal data, for example, but I'm comfortable with it because I can tell when I'm on one of their sites and act accordingly. - Google offers hidden services to site owners (like Google Analytics). Through these popular services, they keep collecting data even though I don't see anything Google-related on the site I'm browsing.

A third issue is that Google's approach to data is different from most other web services (except facebook etc.). Most companies need to store your data in order to provide the services at all. They have a business model that doesn't depend on your data. It just happens that they get to know you as their customer. - For Google, on the other hand, personal data is the business model. They don't offer services and then find that as a side effect, these services give them access to personal information. No, it's the other way round: They need data and offer free services like honey pots so that people give them what they want.

This on the web alone is a threat. But combined with devices like smartphones that reach even deeper into your real life, it's really, really dangerous.