I read all the answers, and I'm now convinced that there's nothing keeping this site from eventually going down that same path.
Death. Taxes. Gentrification.
Death. Taxes. Gentrification.
#1 and #2 cause strong negative backlash. As you would learn from marketing theory, a user that you have personally screwed with will likely cause 20x more damage to your brand than your best users improve it. #3 Locks out a lot of useful expertise. For sure, HN could use a lot more knowledge about programming. So far, this site has attracted lots of people who want to learn but much less so people who already know a lot and have wide experience. We still very much need to be inviting to the latter.
Another point that very knowledgeable people are very opinionated, stubborn, and generally assholes. Linus Torvalds is the stereotypical example. This is not to excusing the behavior, but the correlation exists and we will all have to try not to get so offended and ban people like immature children.
If you try to hold onto the past forever instead of trying to improve then you will fail.
I hope this was useful.
I'd say that a jackass active on the site is more damaging than one banned. If you ban someone, you have only upset one user. If you allow a spammer/troll/flamer/etc to continue unchallenged, you have upset all of them.
The thing with online communities is, this is Web 2.0, your users are your product. The better the users, the better the product. If you have a good community, smart, helpful people will be attracted and will thus improve the community. If you have a bad one, they will be warded off. Scum breeds scum.
Think broken window theory.
The problem isn't with them badmouthing you on your site, it with them polluting your product. If the product sucks, people won't use it, no matter how much you suck up to them.
In fact, it's unlikely to ever be a problem: if your site is small, nobody on a bigger site is going to care about a someone bitching about it, so the story will get buried. If it's large, any site worth its salt will delete a griefing post to avoid the inter-site grief (see: reddit deleting the posts that lead here, mefi deleting pretty much anything leading to LGF)