What the blog topic icons are for:
merit badges Stuart at the Strange Magic once proposed. At one point, I said I was planning to use them as topic icons, and added a few that only made sense as blog topics.
The main reason for these topic icons was so that almost every post had a related picture that could show up when shared to places like Facebook. Also, they provided a quick visual queue on what the post would be about. Tags or Labels do the same thing, arguably, but they are small and at the bottom of the post, as opposed to near the top.
I didn't use them as much as I should have, for various reasons, the most relevant one here being they didn't look as good as I would like. I like these new icons better.
Also, I plan on adding quick visual category links to the bottom of some posts, mostly those involving downloads, so that people can find related material.
Subcategories of Maps:
The Adventure Maps icon is a treasure chest with a map behind it, slightly tweaked compared to the previous version... I tilted the chest to make it more obvious what it is. Geomorphs shows building blocks, Modular Dungeons uses puzzle pieces, but honestly I could have swapped the pics. The concepts are very similar.
Not every topic icon will be for downloads. For example, each of the four main character classes will get its own icon, for posts where I talk about variant rules or support tables. These three are for Fighter (and Combat,) Magic-User (and Magic,) and Thief. I haven't made a Cleric icon yet, because I'm still figuring out how to illustrate the idea. Can't think of a cleric-specific hat. May do a pope hat (mitre,) or maybe a halo.
Other topics still need icons. Monsters, Spells, Psionics, maybe generic Wilderness. The Last-Minute Gamemaster and 9 and 30 Kingdoms Setting topics need icons. So do big projects like Our Undying Neighbors. A lot of these, I just haven't come up with good visual ideas yet.