Potentially open for abuse. Anyone can make a fraudulent DMCA takedown request and Google has to honour it.
Well, no, they don't.
I'm pretty damn sure that if I claimed I was the rightsholder for Thriller and submitted a takedown notice, it would not get taken down.
DMCA takedowns are dicey, but there's not actually an obligation to respond to ones that are patently frivolous and false.
So again, the question is, how does Google define "valid"? It's clear from context that its definition falls somewhere short of "declared valid by a court of law". So does "valid" mean "unchallenged"?
And that brings up the question of how QUICKLY this is going to happen. If a site receives 2000 takedown notices in one day, does its pagerank immediately drop? Or is it given a period of time to respond to the takedown notices? If so, how long?
It's also probable that Google's definition of "copyright removal notice" is broader than simply DMCA takedown notices. I'm thinking it probably includes YouTube ContentID, too -- and as I mentioned, ContentID is a piece of shit that recently took down the fucking Mars landing videos that my goddamn taxes paid for, fuck you Google.
That, really, is a kind of worst-case -- sites getting penalized, immediately and without a window for review, due to a false positive on flawed pattern-matching software aggressively enforcing a copyright claim that the supposed claimant never even made and is in fact frantically trying to reverse.
On the one hand, I'm sure Google's TOS are pretty fucking clear in disclaiming responsibility for fuckups like that.
On the other, seeing as Google is literally synonymous with the service it provides and YouTube occupies an equally dominant market position, a day could come when Google's capricious nonsense makes it legally vulnerable no matter what the TOS page says.