I see traffic as a major signal.
For analytics Google has:
- Google Toolbar
- Chrome (with toolbar)
Those four products give Google the clearest picture of the web available to anyone in the business. More clear than even the large isps like Time Warner and Cox.
How would you apply that data into the algo:
- Pure site traffic
- Traffic patters from site to site could detect intent
- Site patterns after users leave Google
- Detecting problems - like link anomalies
Those are just the off-the-cuff ones. I'll stop right there because only the data could tell you where too go from there.
The only problem with all this? It is not directly influence able by site owners.