Why, why the following questions were not included in перепись населения:
1)How many pairs of doors are in your подъезд?
2)Do you have a metal front door in your flat?
3)How many front doors are there in your flat?

Well, here's my little contribution to this door topic :
1. Steel doors in apartments.
They appeared after the collapse of the Soviet Union so I don't think it's connected with cold winters. Of course, they won't stop proffesionals, especially if they dream of nothing but breaking into your flat but there are some categories of criminals that they will stop. And, as mentioned by Greubau, the default doors open inside thus making it easier to break into the apartment while the steel doors open outside. And the psychological factor, of course.
If you install the metal door and leave the default (wooden) door you will have double front doors (as I do). And this is connected with cold weather (know that from personal experience when I forgot to close the wooden doors ). I am not saying that the majority of people have metal doors but there is/was their relative popularity (at least in Moscow, btw, an indicator of the popularity is the number of firms that offer their steel door installation services and their ads in free advertizing papers).
Also here’s an article about the steel doors, maybe it will be of interest to somebody:
http://www.teso.infomsk.ru/media/publis ... atid=13344

2. Two pairs of doors in подъезд.
There are certainly blocks with them (mine for example ), but I was thinking if I saw blocks that have only one pair of doors and I don't think I can remember anything certain about that (most likely because I wasn't paying attention) but it seems that I saw подъезды where the second door was open or missing. Well, I will be paying more attention to that in the future. Btw, I think here's the case when generalisations are justified to some extent because a lot of apartment blocks built in Soviet Union were built according to the same projects and I guess there were госстандарты too.