Who Applies Post Notices:
We're looking for long answers that provide some explanation and context. Don't just give a one-line answer: please explain why you're recommending it as a solution. Answers that don't explain anything will be deleted. See Good Subjective, Bad Subjective for more information.
This is what is known as a post notice. Diamond moderators and Stack Exchange employees, anyone with a diamond by their name, have the ability to apply a post notice.
In most cases, these are applied in response to community flags. Your answer likely came up in the review queue's "low quality post" filter, due to its length, and some users may have flagged it as "not an answer". Any user with something like 100 or 200 rep can see posts in the review queue, which means just about any user here could have flagged your post as not an answer. Anyone with 15 reputation can flag a post.
In response to flags, one of the moderators likely applied the post notice instead of just outright deleting your answer. The goal, of course, is to give you an opportunity to improve your post and make it valuable to future visitors.
The rest of this is quite long, but I'm hoping it provides you with an explanation and background, from my perspective, of why the community chose to enforce answer quality:
Stack Exchange Sites are Better than Forums:
Most of the answers on the site, due to this rule, answer the question in their entirety, and they also provide context to help askers and future visitors to know why those answers are right for them. This not only reduces the number of answers, and reduces noise, but also helps people decide what action to take to solve their problem.
If we let everyone post random one-liners everywhere, how useful would 15 to 20 answers on the site be that just didn't explain anything? What it would do is reduce the material to nothing but guesswork for those seeking answers.
Are we discussing Short Answers, or how the community came to this conclusion?
As Yannis said, if you want to rehash the discussion, let's do that. So far, you've just complained about the process of coming to this conclusion months ago and seem to be focusing on that point and trying to call Yannis out for being inconsistent.
What is your argument for allowing these short, one-liner answers? How does this add value to this site? How does this make Workplace SE better than some random snapshot on the Internet where someone posted something vague for future visitors to try and interpret years later? This really isn't what Stack Exchange is about, and quality is why this Q&A platform is so popular.
Short Answers Are Not Bad:
As far as short answers go, I'll take a different approach than the people who say short is bad: Short answers are not bad, and an answer shouldn't, in my opinion, be removed simply because it is short. However, short is a symptom of something possibly being amiss with an answer. Either it doesn't answer the question fully, or the answer doesn't explain why it is the right course of action, in most cases.
In some cases, short answers could indicate a problem with the question. Perhaps someone should leave a comment asking the user to provide more details. Maybe a skilled editor can help clarify the post. Possibly, the question just needs to be closed as not a real question.
In many cases, short answers mean there is a problem somewhere, but not always.
Interpretation Of Your Answer:
As a result of my stance on short answers, I've read your answer and reread it again, trying to find a reason to take your side and argue for it to be undeleted. The conclusion I come to is that I'm not 100% sure you answered the full question, but the part you did answer didn't really need explanation because it was so obvious.
You did addressed the issue with being fired, which is a concern for the applicant, but you didn't address whether or not it would be an issue to an employer if the applicant consciously quit after 1 year.
You didn't answer this:
"what would you prefer to see in a job applicant? Exactly one year at the last job or 13-14 months?"
And you didn't answer this:
On the one hand, if I quit after 'one year' it seems more likely to be a conscious choice and something I would have given consideration and planning before doing.
If you're applying to new jobs while you still have a job, this shouldn't matter, since you obviously haven't left because you were fired.
So, what I get from this is that employers won't think you've been fired if you still have a job, obviously. This makes sense, and again, I agree no real explanation is required here. However, the asker was also concerned that this might make him/her look like he/she planned to leave the job after 1 year, which might look bad to an employer expecting people to stick around and dedicate themselves to 3 or 4 years.
Again, you didn't address that point or explain why this isn't an issue.
Why Context Matters:
To me, it's self-evident that if you're offering an answer to a question that you're recommending it as a solution because you think that's what the person should do. So insisting that that be included in the answer is just silly.
So who are you? We don't know you. I really just met you online a few days ago. You seem like an intelligent person, and I even watched you answer an interesting ActionScript problem on Stack Overflow a few nights ago. You even said ActionScript wasn't your area of expertise, yet you still solved the problem. ;)
However, user1234 doesn't know you. How does user1234 and the countless future visitors to your Workplace post know that the information you give is useful and correct if you don't provide any context to demonstrate it's correct? To user1234, you and I are both just random people on the Internet, so without explanation, it's difficult to tell why a solution is a good solution.
In short, I think you're misunderstanding what the community means by "provide some explanation and context". The community isn't asking you to say "I recommend this explanation because I think it will help you", because you're right, that is silly. Instead, the goal is to explain why on a deeper level. Share your knowledge. Why do you think what you're recommending is helpful? Because you say it is? How can you explain this so that people reading your answer learn from it, instead of merely thinking "I should do this because Amy says so.".
Remember, there's no compiler here to check to see if a solution is good or not. Sometimes, the outcome of an answer on the Workplace may not be clear for months or years, so there's no immediate way for someone to tell if your solution is the one to go with. Hope this helps!