There's actually more than one Quality Score. There's the keyword quality score, and for the Content Network, I think it's more of an Ad Group quality score (as the individual keywords don't matter as much) There's also an Account Quality Score, where they assess your overall account. That's what kicks in when you start a new campaign. If they don't have any other performance data to go on, but you have a good Account Quality Score, they'll give you the benefit of the doubt for a while based on that, until they find out how your new campaign is actually doing. If your overall Account Quality Score is bad, you'll be Google-Slapped as soon as you start a new account, with $1 to $10 minimum bids, and have to work your way back from there.
And there may be other quality scores that nobody outside Google even knows about.