I would like to make a few statements on each of the primary three points.
Description: Giving a good description of the setting is of course highly important for any thread and, alongside it, any story. However, one thing I would like to note is that, like their characters, players do not automatically have access to all information about what is happening in a plot. Rather, I find it important to give players options outside of the most obvious, in order to reward any potential sidequesting they may decide to do.
Simple example, you find an old woman standing in the park looking up at a tree from which a faint meowing can be heard. Looking around the bright green meadow, one can see all manner of people, people of all different ages and backgrounds, talking excitedly as they pass each other. There is an older, Japanese businessman relaxing near the large, central pond. Near him are a couple of teenagers, likely out on a date.
Action: Along with Ene here, I believe I would like to sharply disagree. While it is important to make sure something is happening in the thread, something the characters are aware of, it is equally important NOT to give players an obvious goal, or at least an obvious path to reach their goal. That way the story has many more options to unfold. In essence, I don't want to tell the players why they are here, I want the PLAYERS, and by those means their characters, to have their own reasons for being there. Do a good job with the other two factors, and this should become by far the LEAST important.
In my previous example, the action here is the meowing from the tree. Its probably a cat, so most players will decide "oh, this is the problem that needs to be fixed. Lets go do that."
Reaction: In my eyes this is the most important of the three, even more so than describing the setting to begin with. If these people are going to act out their part in a world, they need to see how exactly their actions change things. In essence, what I mean by "reaction" is how the setting responds to a player character's attempts to pursue their own goals. Regardless of what these goals are.
In my previous example, the only "action" happening is the old woman who needs her cat, yet the players could easily do things like talk to the businessmen, eavesdrop on the couple, stand near the lake, or even steal the cat for themselves. All of which should be considered valid actions and responded to appropriately, where the story goes from there depends on what each player did.