Some ads are intrusive in the way that they highjack my thought processes. I'm reading about hard-drives, I don't want to think about science fiction right now,

Some ads are intrusive in the way that they highjack my thought processes. I'm reading about hard-drives, I don't want to think about science fiction right now, and vice versa. So unobtrusive also means matching the content in spirit. (I might be fine with related tech, for example.) At that point there shouldn't even be any point in user preference tracking any more. I even click these ads on occasion if they seem trustworthy (another important point - there is still too many 'You won a car!'/'Lose weight with one simple trick!'/'Doctors hate him!' kind of ads).