> I don't know that there's a one-size-fits-all category of "monoculture." We know the extreme example, IE6, but does webkit deserve to be put in that bracket?
An open source monoculture is better than a proprietary one, for sure. But it's still bad.
Aside from security issues, there is the concern for standards. Standards are meaningless with a single implementation. And without standards, making additional implementations is extremely hard. Right now, it is possible for someone to write a new web browser with new technical improvements - they would implement the standards. If WebKit is the new IE6, then a new browser would have to do "what WebKit does."
> For one it's not solely influenced by one "culture." Adding Opera developers adds to the mix, doesn't it?
Not much. Opera is going to be shipping a rebranded Chrome - they aren't even taking WebKit, they're taking Chromium. Opera will differentiate through UI, it appears - hard to see what else they can.