This sounds promising. I'm worried about two things:

1) Will a minimum number of endorsements lead to groupthink? Will a substantial, polite but unpopular comment still get endorsed?

2) If this works initially and improves comments immediately, could it have negative long term affects? Will new users find it confusing or intimidating? Will people comment less knowing that others are less likely to see it?

And one question:

Why not have comments with 0-3 upvotes only visible to people with 1000+ karma? Isn't that the same thing as the endorsement system?

Worthy experiment, regardless.