One subtle bit of annoyance is how the floor and carpet tiles get drawn in various "edge" cases ;)
In the first image, I'm building green carpets underneath my hydroponics stations, but the game is drawing them as though they were black.
In the second image, the floor tiles on the edge of the geothermal plant are drawn using the "rough stone to tile" transition, but the game won't let me smooth the stone underneath it unless I first demolish the geothermal plant. That's quite a way to go for some visual aesthetics :D
In the third image (a common one I observe), there is no carpet on the door tile but there is within the hallway. It leads to visible dirt bleeding across the threshold, like a really lazy incomplete carpeting job. Doors should count as walls in terms of what they cause neighboring floor tiles to draw as.
Hydroponics
[attachment deleted due to age]
Geothermal
[attachment deleted due to age]
Door threshold
[attachment deleted due to age]
1. Is working fine, it's not a black carpet transition, it's just the edge shadow all buildings have. Unless I'm missing something.
2. True issue. Not a bug, but a micro-annoyance.
3. You can place whatever floor type you want in a doorway. I believe this is correct. If you want a doorway to look a certain way, add the appropriate floor. I don't think there's an issue here.
Not-quite-bugs all around, so you'll understand if I thank you for reporting (with pictures!) and close this.