When I first learned today about this change my knee jerk reaction was the same as with most people commenting on this---it will be horrible, it will enforce groupthink, it will stifle discussion, and so on. But after some thinking, I realized it's just a knee jerk.
The thing is, HN doesn't have a problem with rudeness or bad comments that existing voting can't solve (as far as I can see). But, I do think that HN has a problem with the amount of comments on popular submissions, and of a huge amount of comments which are nominally OK (i.e. not rude, not wrong, not trolling...) and therefore cannot in good conscience be downvoted, but whose information content is near 0. With low total amount of comments, this wouldn't be a problem because most of these kinds of comments are a consequence of there being a non-manageable amount of total comments for the speed at which content goes through HN---most people don't have the time to go through the whole lot and check what has been said/pointed out/asked, and that leads to a lot of redundancy. This is very exhausting to read through, and really is a problem of signal to noise ratio, but the "noise" on HN is not the stupid or the rude (those get blasted away quickly), it's just the well meaning, decent but redundant comment made because one has something to say but can't be bothered (rightfully so when there are 300+ comments already!) to check if it's been said already.
A recent example: https://news.ycombinator.com/item?id=7427542 This was just horrible! Comment after comment telling him the exact same thing (change your CV) with almost the exact same specific advices. I went through the whole thing because I was interested and wanted to see what people have to say about this, but I was completely exhausted at the end because there was so much redundancy.
Why do I think pg's plan with pending comments would improve this? The first order effect would probably be a dramatic decrease in number of comments. The second order effect would then be that it's much, much easier to go through the whole thing and see if there's any new information you can contribute to the topic, which, if we are sane people, will lead to a dramatic decrease in noise (redundancy) (I say "if we are sane" because I take it that a sane person won't repeat already stated ideas just for the sake of repeating).
This is why I'm currently strongly in favour of the way the idea was presented originally with some tweaks that have been mentioned in the original thread yesterday. I see now that it's off by default and only per item tree, thus only serving as a weapon against rudeness and flame wars. As I don't think HN has that problem so I don't see the point of the whole thing as it stands now. I would much rather if the original form was given a chance with the mentioned fixes and attentive parameter tweaking.
As for fears of "echo chambers" and such, I don't see that currently on HN and I'm willing to give people here the benefit of the doubt that they would in aggregate do the right thing. If, on the other hand the community is so bad to end up using this as a tool for censorship then I don't think it would be a shame to have it fall apart anyway. So I don't think there's anything to lose. Useful back and forth discussions would still be able to go on with parameters reasonably tuned.
Also, I notice that a lot of people mistakenly think that 1000+ karma means your posts get auto-approved. But that's not the case, it requires 10k+ karma! Realistically, most people will never reach that even with the system as it is now, so that's completely irrelevant. Users with 1000+ karma would not be in any particular commenting advantage if the proposed change went through.