Awesome as always, Markavian!!
The first obvious comments on the slice visualizer:
The icons you show on every slice are still shown as viewed from the top. This is more obvious when seeing walls. Of course I realize there's no way for you to change walls to display "sideways" since they are basically 2D sprites or icons. A quick fix tough could be to replace wall icons and show them as solid rectangular chunks of the appropriate material color as if they where smoothed rock. This will give more the proper illusion of seeing walls from the front.
The other thing that doesn't look so good sideways is the surface water. When you see water from the top you can see the same water level or icon on the +0 level and the +1 level (in a darker color). This reflects badly in the slice view because you'll continue to see the darker water icons on the level where there supposed to be no water. The same goes for lava. This maybe could be fixed by hiding the water and lava icons that are darker in color.
Anyway those are minor annoyances.
By the way. Any thoughts on integrating the recently released awesome 3d visualizer by sinoth? Maybe you could just let people add 3d fortress screenshots from the visualizer as an addon to the POIs.
Maybe you could make your visualizer more like google maps by adding the POIs into the maps as little icons that the users can click and will spawn the POI comment. And inside the comments the viewer can show either a 3d picture of the fortress or even a cmv movie.
Anyway, these are not requests. Just ideas for future features :p