Well unstable as the USSR was, at least (for the formative period) the neighbouring countries were either friendly socialist states (Germany, Hungary) or stable, non-menacing democracies (Czecho-Slovakia, Sweden). The fledgling democratic traditions had the time and space to develop in the 30s and 40s. I have always thought that the outcome of the Great War and the following Revolutionary period was very beneficial and, actually, a quite lucky one for the USSR.
Maybe, if the USSR was threatened by aggressive, imperialist or racialist states early on, the development of the Workers State could have been perverted towards authoritarianism. A Second Great War (?) or a serious threat of such, against something like a league of European monarchies presumably could have had such an effect.