How does PLOS ONE maintain its impact factor? The PLOS ONE journal has an impact factor over 3. The specialist journals in my field top out with impact factors of about 3. As impact factor is in essence a measure of citations per article, it seems surprising that a journal like PLOS ONE that prides itself on not making subjective judgments on things like "importance" can maintain a higher impact factor than journals that prioritize "importance".

I have only cited handful of articles published in PLOS ONE and looking at what has been published in PLOS ONE in my field, the majority of things look "unimportant." It seems like in my specialty that PLOS ONE publishes a higher percentage of "unimportant" work than the specialist journals. This of course could be due to sample size and my own bias.

I am curious about what drives the impact factor of PLOS ONE. Are there specialties for which the PLOS ONE impact factor is low compared to specialist journals? Are editors and reviewers of specialist journals particularly bad at identifying "importance"? Is there something else?
There are, I think, two distinct factors at work that may help explain some of your puzzlement:


Your field's impact factor is not academia's impact factor. For example, society journals in my field have an impact factor of ~ 5, and some of the big names for very splashy studies have impact factors ranging from 20 to 56. Depending on the balance of fields submitting to PLOS ONE, their impact factor may be coming from more cited fields.
Long-tailed citation papers. Impact factors, like many averages, are susceptible to long-tail effects. PLOS ONE is an open-access journal, and a highly visible one. It's possible that the occasional highly accessible generalist paper makes it there, and yields a large number of citations as a result, pulling up the overall impact factor.


Both of these are helped, in my opinion, by the lack of review for "importance" - beyond your suggestion that this does result in less important papers ending up in  PLOS ONE, it's also a benefit to papers that don't quite "fit" in highly specialized journals, but may still be impactful.