Hey! I only just noticed that the Wang functionality was merged into the terrain tool a couple releases ago, and the combination is definitely a vast improvement! I’ve been playing around with it a bit, and here are some obscure snags I’ve run across. I intend this not as criticism, but as a list of weird stuff people might try to do in practice. 
One: It’s not possible to draw a single unconnected tile. This is no big surprise, as there is no way to assign a color to the center of a tile. And it makes sense for the original design of the terrain tool, which was for transitions between different terrain on a top-down map. But for a platformer, where most terrain is interested in transitioning to empty space, single tiles make perfect sense.
Two: I don’t know if there’s a way to enforce decorative edging when transitioning to empty space. I have a blob set of dirt tiles, with grassy bits along the top edges, assigned like so:
If I draw with the green terrain, I can make a row of connected blocks, but of course nothing taller than that. If I try to draw with the red terrain, I get some odd suggestions:
It will never propose two connected grass tops, nor propose the grass itself. I was expecting something like this:
I also can’t draw the dirt version of these tiles and then use the terrain tool to place the grass on top:
I don’t have grassy-topped versions of every possible open-topped tile, so some arrangements really are impossible to satisfy (which has been a problem with my automap rules for this, too). But this seems fairly straightforward, so I’m confused as to what’s going wrong. My best guess is that empty space is special-cased to only work when there’s a single terrain in a set?
Three: I tried out an arrangement of generic non-trademarked plastic building bricks, like so:
There are no top and bottom edges marked because there’s a seam there, so any tile can go with any other tile. (It could be nice to prevent two blocks of the same color from touching, but I can’t see any way to encode that without allowing an edge to be multiple colors.)
The terrain tool seems to get a little confused about this. If I drag from left to right, I sometimes get broken bricks:
I’m guessing what’s happening here is that the middle block was previously red, and I’m trying to place more red as tile 4, but there is no red/red tile, so Tiled has to change tile 3. It randomly picks blue/red, but that would make tile 2 be blue/blue, which doesn’t exist, so it has to change that, randomly picks yellow/blue, and refuses to go any further away from the cursor.
That makes sense. But I don’t specifically want to place red; I just want to place something. I thought that was the general idea behind Wang tiles, but I don’t think there’s a way to draw “any terrain in this set”.
If tiles’ centers could be colored, I could assign the center of all of these tiles to a fifth color (white?), and then draw with that as a workaround.
Four: Similarly, floodfilling or shape drawing with this non-Lego set has curious results:
Not quite what I had in mind.
Five: I do also have some decorative top-edge tiles for this set, but the correct one to use depends on the edge colors and the color of the brick below, and I have no idea how I could possibly encode that. A correct arrangement would look like this:
I can make automap rules for this easily enough, of course, but those can’t erase decor that’s no longer necessary.
Six: Oh, and I’d love an “edit tileset” button or similar for the map’s terrain list, similar to the tile grid!