There's a HUGE difference between trusted users on Stack Exchange and trusted users on a forum, and even more so with the current implementation.
On Stack Exchange users are expected to answer really tough questions with really solid, well-written answers if they want to get any upvotes. That requires skill and dedication, and the scale of their user base means that it's really likely that somebody would have noticed some spam links somewhere.
And it's worth noting that Stack Exchange didn't just make all "trusted" users have followed links. Read the meta post. They did it for Posts that are considered Reputable...
Starting today we will be removing nofollow on links within posts that hit a high enough threshold to be considered reputable. The details will remain somewhat vague at the moment to discourage gaming of this feature.
On a forum, people post conversational things that are sometimes Liked. The most likes are usually reserved for a funny picture and a witty retort. Not exactly the stuff of high-quality link building.
On Discourse, all you need to do to be a "leader" is post 10 replies. (and yeah, visit and "read" a bunch, but that could be easily automated with a bot.)
A clever spammer just needs to get involved in every DIscourse forum enough to post 10-20 times, get some people to hit Like once... and then run a bot that checks the page every day for 100 days. Then they can post links without nofollow.
I'm going to dramatically change the requirements for becoming a Leader on my forum.