Sure, it's nice to hear about all your great technology, and how many users you have, and your latest huge funding rounds...but none of that will make me wanna get up in the morning. After a few months on the job the only thing that really matters to me is who I'm working with, not necessarily what I'm building. So, I look for job ads that focus on culture, core values, and team-member health. I like to see that the company is proud of (and values) their people as much as their stack. And I like to see team members who are active in the open source community, who attend meetups and such, and seem to have a nice work/life balance.
I try to picture myself there, and I figure: "If I take this gig, in a few months I'll likely be the average of the people who are already there". So if the existing team members seem happy and healthy, that's a good sign. If they seem overworked and depressed, I'm going to stay away.
Along the same lines, asking for a JavaScript developer and then tacking on "must have great UX skills" is a huge red flag. Does the company really not know enough about UX that they think it's appropriate for the front end developer to sort of figure out as they're working?
Like others have said, using words like "ninja," "rockstar," "guru" is an immediate turnoff, although that's becoming less common. Using cutesy trendy references in an attempt to be cool, or overplaying the "cultural fit" aspect is also a little worrisome. Like, "okay, I understand that everyone is TOTALLY AWESOME and loves to drink beer and play foozeball, but what about the 401k matching?" If you want to hire a bunch of out-of-college brogrammers, that's fine, but I'm not going to respect the company, or want to work there.
Clear, concise, detailed description of what the job actually is and perhaps an example of what the day-to-day activities and responsibilities are.
Similarly, be detailed about the benefits -- they tell you a lot about a company. A job description that mentions family leave and 401k matching makes it seem like they actually care about employees. "Great benefits!" sounds like they're hiding something.
Essentially, the job posting should just seem written by someone knowledgeable about the field (no questionable language, no using trendy but kind of not-what-you-really-want words like "CSS3" instead of "CSS"), professional, detailed, and appealing. I mean, what would you want to see on a resume?
I'm most interested in a place that gives a shit about their employees. Work life balance is huge. Pay scale is huge. Rewarding hard working employees is huge. If the description looks like something that would entice a 22 year old college grad, I'm probably not interested.
That's the number one thing I look for, and if it's not there, I move on. If you want to attract good people, give a salary range. Don't tell me it's "competitive" (which means it isn't).
2. Remote
If it's not remote I'm not interested.
3. Techs
Ideally this should be a very short list. For instance I wrote a job ad a few years ago and asked for just Rails and jQuery. If you tell me you're looking for XML, JSON, REST, or APIs I'll know you don't really understand that those techs are puny compared to ones that require lots of experience.
4. Professional/Career Development
Are you offering a job or a career? What opportunities for growth do you offer? Is there a training budget? What about training for non-technical stuff? Do you send people to conferences? Do you give people time to write and deliver talks at conferences? Is there a technical track with comparable compensation to the management track? I know startups don't like to "do HR", but if you have thought about this you will really stand out.
Compensation sends a stronger signal about the quality of your company culture and the seriousness with which you treat work/life balance, than any words you can speak or put on paper.
To add to this, a list of a few keywords roughly in descending order of unattractiveness - for the most part I would pass on any listing that contained any of these: ninja, rockstar, "work hard, play hard", scrum.
For closing the deal, however, I look for good pay (e.g. don't offer me equity in lieu of money, especially when some quick research reveals you're also receiving candidates from recruiting agencies) and I look for a good fit (e.g. culture/policy of reasonable work hours, I'm allowed to - and encouraged to - hold intelligent discussions with the team rather than wasting my time w/ HR bureaucracy or googleable trivia, etc)
Having entry level positions is a good thing, as long they don't require 2-3 years of work experience in your stack when you are hiring new grads. That's basically a red flag saying you want to find somebody who's cheap and not give them any training. But I definitely like if a company is hiring for a range of experience levels. If you are only hiring senior people, then where is the opportunity to grow in a position?
Jobs that want people who can do the job are preferably. If you only want people who have already done this exact job, you are asking people to move laterally, not up.
I love a description of common tasks that I would be expected to perform. I like to know how people communicate (multiple in person meetings everyday are a red flag though).
Having looked at my github code, or at least at the readme files, is nice. Don't ask me if I've ever done angular when I have an angular app on my resume with a working example. btw, I do Angular sure, but I don't respond to anything that requires photoshop skills.
Jobs that list how their interview process works make me happy.
Other than that, any mention of good coding practices (automated testing, continuous integration) or technologies that show the company is keeping up in the field and not getting too bogged down in a lot of legacy code.