That's a pretty fair criticism, and I have to agree with everything that you write here. But I don't see any way around that. I can't imagine a general interest site could possibly function like HN.
About 18 months ago I stopped posting regularly on HN after my former account was downvoted into a type of purgatory that was impossible to escape from. I still read the interesting articles and the occasional interesting discussion, but something I discovered in the intervening period of participating in unmoderated/uncensored forums is that trolls make you THINK.
There are some exceptional trolls out there on the interwebs, some of whom do it as a day job. They post articulate but specious arguments that are very difficult to debunk unless you do meticulous research, double check your calculations, edit your reply for succinctness, remove any possible semantic ambiguity.
I think there is an argument to be made for the stimulating effect of trolls on the rigour and effort of the opposing participants. I agree however that trolls can and have destroyed valuable forums of discussion, but so has lack of interest.
>something I discovered in the intervening period of participating in unmoderated/uncensored forums is that trolls make you THINK
Some trolls make you think. Some trolls only hope to destroy online communities, and they are usually very good at it. It can be very hard sometimes to differentiate between the two.
Also, it's entirely possible to make people think without trolling (and I know you weren't claiming this). For example, your response to my remark made me think without being trollish. I personally prefer to participate in communities that promote your type of provocative comment, and not in ones where trolls are given free reign. But that's just me.
I agree with what you say. There is a fine line, however, between censuring trolls and suppressing dissent. I think anonymous downvoting and no right of reply, risks the latter to gain the former.