There are all kinds of motivations to post a negative review, and not all of those motivations map "reliably" to accuracy. Leaving a job can be traumatic for employees for reasons that have nothing to do with the company's culture or behavior--a disgruntled employee fired for cause has a strong motivation to leave a bad review, but that doesn't mean it is reliable.

Glassdoor has the same pros and cons of any review aggregator. It is susceptible to gaming (perhaps uniquely so, like Yelp, because the companies being reviewed are also Glassdoor's advertisers). But it provides a directionally accurate view of a corporate environment given a large enough sample size of reviews--and as with any review, more credence should be given to examples that are clear, detailed and well-written.