At least it's a fairly descriptive one. If you can come up with something better, please, by all means start using it in conjunction of current longread threads. A good (or improved) idea will eventually get picked up.
I for one like the ability to find thorough articles, regardless of their topic by a well known tag. -shrug- At the moment that tag appears to be "longread". Now, one CAN argue what should be the minimum length for an article to consider it a longread. (My background stems from magazine freelancing, so I tend to count characters and not words.)
As a first apprpoximation: if it takes longer than 20 minutes to read the article, it might qualify. If it takes longer than 45 minutes, it most certainly does. As to how many will give up before reading such a monster through, is another question...
This is the first time I have seen it. I like it, I see it as an antonym to whatever we call those "top ten weird science facts" posts. It signals that here is an author who is trying to take me seriously.