There are two complaints that I see as supposed objective reasons for old school folks' easy complaining bout how it is not a real journal. First, that they simply publish "too many papers". It was 23,468 in 2012. This particular complaint always reminds me of
which is to say that it is a sort of meaningless throwaway comment. A person who has a subjective distaste and simply makes something up on the spot to cover it over. More importantly, however, it brings up the fact that people are comparing apples to oranges. That is, they are looking at a regular print type of journal (or several of them) and identifying the disconnect. My subfield journals of interest maybe publish something between about 12 and 20 original reports per issue. One or two issues per month. So anything from about 144 to 480 articles per year. A lot lower than PLoS ONE, eh? But look, I follow at least 10 journals that are sort of normal, run of the mill, society level journals in which stuff that I read, cite and publish myself might appear. So right there we're up to something on the order of 3,000 article per year.
PLoS ONE, as you know, covers just about all aspects of science! So multiply my subfield by all the other subfields (I can get to 20 easy without even leaving "biomedical" as the supergroup) with their respective journals and.... all of a sudden the PLoS ONE output doesn't look so large.
Another way to look at this would be to examine the output of all of the many journals that a big publisher like Elsevier puts out each year. How many do they publish? One hell of a lot more that 23,000 I can assure you. (I mean really, don't they have almost that many journals?) So one answer to the "too many notes" type of complaint might be to ask if the person also discounts Cell articles for that same reason.
The second theme of objection to PLoS ONE is as was recently expressed by @egmoss on the Twitts :
An 80% acceptance rate is a bit of a problem.
So this tends to overlook the fact that much more ends up published somewhere, eventually than is reflected in a per-journal acceptance rate. As noted by Conan Kornetsky back in 1975 upon relinquishing the helm of Psychopharmacology:
"There are enough journals currently published that if the scientist perseveres through the various rewriting to meet style differences, he will eventually find a journal that will accept his work".
Again, I ask you to consider the entire body of journals that are normal for your subfield. What do you think the overall acceptance rate for a given manuscript might be? I'd wager it is competitive with PL0S ONE's 80% and probably even higher!