Off the top of my head, here's a few issues I have with this:
1. Lack of transparency - I bet most people that don't read tech sites like this one don't even know about it. Nobody knows how it works, or how to turn it off, yet it is in the wild for ~0.25% web users.
2. Who decides which website are classified what way? Website classification based on its URL is hard.
3. Can people remove websites from their 'history'?
4. What about changing interests, or curious people? Will sites 'expire' from your interests?
5. This is all tied to Google, and they're trying to make it a web standard - what happens if they drop it like they are wont to do with things?
Anonymity is a distant dream unfortunately, given how effective browser fingerprinting is. This is just another datapoint to add to the mix.
Biting the hand that feeds IT
