I didn't have long to try it out but I tried to blueprint a building more similar to those where I live and I had some troubles, both related to that and in general, in order of importance:
No quit button?
When I placed the roof (by accident) I could no longer see the floor I was trying to place objects in, or the roof was that floor? (what about attics?) and I could not find how to delete a roof...
No basements? Basement shops and car parks are pretty common, if not ubiquitous here, even the centre of my small town has the main supermarket underground in the basement of the biggest building with the one next door having bar and restaurant in the basement...
No indoor windows... I tried to build a mixed use building with apartments above and shops below; the two ground floor shops were separated by a hallway that leads to the stairs to the apartments (very standard design here) but I wasn't able to make the walls between the shops and the hallway into windows as they would normally be, though I was able to use the same doors for inside the building and outside the building which was good :) - if I can't make indoor windows I don't know how I can make "galley" windows either... a window from the kitchen to the dining room to pass plates and dishes through (very common in older houses).
When you select delete mode and then select a type of deletion the type submenu disappears which was unhelpful when I had to delete my building because I made the wrong initial zone... talking of which:
It was not entirely clear how to make a mixed use building - maybe if the user tries to add a floor with a different zone type then automatically make the building mixed use? (But better also to put up a message saying this will or has happen(ed)?)
Just my first few thoughts but other than the above I found it very good indeed and will try to play around with it more once I have more time to spare, apologies, I have to go now...