For threads/posts in general, I think I agree with the sentiments expressed by ravells and Midgardsormr in the professionals vs amateurs thread. For the creative contributions, I think there are two main criteria that I use to determine what is worth a new thread/post.

The first is Place. Although mapping to me is a diagramming process (deemphasizing non-critical information, enhancing important information), the spirit of this forum seems to be mapping distinct places, real or imagined. With the amount of tools and resources that others have gathered here, it is suddenly a much easier process to develop an understanding of a new or existing place. Very exciting. (Admittedly, I have only been poking around here for around a year or two, so perhaps more non-place mapping has occurred in the past.)

The second is Time. I have found that it is tough to understand maps (or diagrams in general) if a certain amount of time/effort has not been put in already. Before I started posting here, my maps never needed to reach the 'easy for others to read' stage because I was the whole audience, and therefore understood lines and shapes without the need of lightweights or legends. The tricky thing with time is that some have already invested time into the overall process instead of the map itself. Since I have not yet invested much time in developing these processes, 20 hours of my time may be equivalent to 1 hour of others with more mature map-making processes.

As an example of something of mine that did not make the cut:
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A diagram of a process that I spent about an hour on. This illustration grew out of a conversation in another thread about the ability of Roman bridges to span wide rivers. After looking through quite a few on Wikipedia I started noticing that some of the bridges had islands near one shore under the span of the bridge. I finally got a hint of why in the article on Band-e Kaisar, which said that the island is man-made, since the river is rerouted through a canal while the bridge foundations are built in the riverbed.

Band-e Kaisar - Wikipedia, the free encyclopedia

In the end, however, the illustration fails both tests. Although its dimensions are inspired by Band-e Kaisar and Puente Romano (near Merida), it is diagramming a theoretical building process, not an actual place. Also, I only spent about an hour on it, and given more time there are a number of easy changes that could be made for the sake of legibility (starting with the Title, which should probably read "How I Imagine Romans Built Some of their Bridges").