Every possible feature must be examined with a few variables in mind.
- How does this benefit individual users (consumers)
- How does this benefit all users (community)
- How does this contribute to the goals of the site
How does this benefit individual users
This can be subdivided into three categories: Ignorers, Problem Users, Bystanders
This group benefits the most, obviously. There is some user that they do not want to see anymore. Those comments will disappear, those answers will disappear, and those questions will disappear. The minor consistency problems with this have been illustrated already. But those could be mitigated by simply having an 'IgnoredComment' pop-up of some kind. Configurable, of course. Like the ignored tags.
The downside is that this user could potentially miss out on a good question or good answer. But it's a huge community, so they probably wouldn't be missed. The ignorers will end up being more productive and happier as a result.
These users are annoying. They're not offensive, otherwise their comments could be flagged as offensive. They are not spamming, otherwise their comments/questions could be flagged as spam. The behaviours that they are exhibiting are simply annoying for whatever reason. The moderators cannot do anything because they are not breaking any rules. So they simply continue to exist and make life worse for those around them.
Once they are being ignored, they have no way of knowing it. They are not being told to get better, they will not notice that they are being ignored. There will be literally no indication to them that they are expected to improve. Furthermore, if someone does tell them to improve (someone who has helpfully not put them on ignore) and they DO improve, they will still be on ignore lists, which means that their past behaviour precludes them from fully participating ever again. As one poster points out: being NEW is considered crime enough to put them on the ignore list.
This also means that users are punished for disagreement. I am someone who believes that many architecture decisions are complex, and therefore there can be many contradictory but independently correct solutions. This means that there will be arguments. I will downvote and comment on questions, and the author will clearly disagree with me. If that user gets frustrated with me, they could just say "I'm ignoring you!" and then feel smugly superior, like they won the argument. In fact, they are simply running away. They have no convinced me of their opinion, they have not convinced me to change my behaviour. Especially on meta, where discussion (heated sometimes) is encouraged, this feature would be counter-productive. You would lose out on voices of dissent. I'm not saying that these people are right, but if you just white-wash and convince yourself that no one disagrees, then you are LOSING out on valuable input.
Furthermore, if there is as few as one incident, they can be ignored. And there is no way to redeem yourself. You are just gone. This goes against the point of SO, where content is judged individually, and the votes speak for themselves.
The Problem Users who, as I've highlighted, are not necessarily problematic at all, will have a much much worse user experience, with no guidance as to how to improve.
Bystanders are everyone else involved. If the ignorers start ignoring people, the bystanders still have to see them. But, there will be fewer people actually trying to help those problematic users. It means that for the bystanders (the bulk of the community) the general product will be worse. The annoying people are still being annoying, but there are fewer people reporting them. If they attempt to answer a question for a user who is on many people's [ignored] list, then there will be fewer people to comment and provide feedback. If they edit a bad question and make it better, this will also be [ignored] by those ignorers. Ultimately, it means that the bystanders will end up seeing less information from the good users, and more information from the bad users, because there are fewer people trying to keep the content-to-noise ratio down.
For bystanders, the net result will be bad.
How does this benefit all users
As I said in the bystanders section. This feature is anti-community. It encourages us to ignore problems rather than try to solve them (an interesting position on a problem solving website). The bad users are still bad, but no one is going to make them better. The reason this community is so strong is that it takes good programmers and makes them better, and it takes bad programmers and makes them into good ones. Those programmers will then become huge advocates who will contribute strongly to the community.
But that won't happen anymore. We are writing off people who need improvement. We are also writing off people who have abrasive personalities, despite the positive things they could contribute.
Another aspect to consider is the effect on new users. I'll illustrate with a trivial/silly example (please don't extend this past the purpose I'm bringing here) If you live in a messy apartment, but you just ignore the mess, and live there just fine. Every new person you show your apartment to is going to comment on the mess. But you don't notice anymore, so you think it's fine.
New users are going to find that there is a lot of spam, and a lot of annoying users. This will cause them to turn around and leave. Yes there is an "ignore user" option, but they won't know about that initially, and they won't bother with trying to get value from the site, when it is clearly just overrun with spam.
How does this contribute to the goals of the site
The goal of the site is to be a repository of information. That means, taking common questions and putting them in front of as many people as possible, and letting the best answers be decided by the community. Adding the ability to ignore users means that the questions (which could be useful to everyone) will no longer be seen by everyone. That means that users who could have provided incredible answers that were read by everyone will now not do so, just because the question asker did not meet their personal standards.
Ultimately, this feature does not contribute to the goals of the site.
Benefit to individual users: Ignorers: Positive
Benefit to individual users: Ignored: Very Negative
Benefit to individual users: Bystanders: Negative
Benefit to community: Negative
Contribute to site goals: Mostly Negative
Just move on. If their comments annoy you, move on.
If their comments are offensive. Flag them.
If their questions are spam/offensive/argumentative. Flag them. Close them. Report them.
Lastly, (for those with the strength of will) engage them. Try to help them. Why are they argumentative? Are they naturally douchey, is there a language barrier, do they have an aggressive oratorical style? Did their wife just cheat on them with their best friend and that bitch wants my car now too?
I can't rule any of those things out, and I really don't care. I just want to contribute to this community as much as I can.