Long-form content is a big part of Hubski culture. The primer mentions a focus on discussion and in-depth debate, thought-out comments are encouraged, the tagline is "A thoughtful web" - and the #goodlongread tag is one of the best examples of this. I think that with a lot of new people just joining the site, interesting lengthy posts are good ways to bring their opinions to the floor and to add them to the community.
From now until next Saturday, people who want to participate in this round of #tagoftheweek should make posts that use both the goodlongread and tagoftheweek tags. The person with the most shared post at the end of the week chooses the next tag.
Cheers!