My friend Dan asked this question on TMP. I think it an excellent one, bringing a lot of thought, at least on my part. Once upon a time, I used to fuss about with a science fiction roleplaying game called Traveller. One of the things I liked about it was the way everyone who played it had a slightly different version of their universe. They even had - I suppose it still exists, though I haven't had an interest in years - an acronym for the concept: IMTU, or "In My Traveller Universe." I think the same concept can be carried easily across into Victorian Science Fiction as "IMVSF". And so, these are the things that I think define the genre, and my own personal takes on them.
First, the time period. Obviously, the mere name of the genre's inclusion of the term 'Victorian' gives you a range. It's quite a large one: more than sixty years. Personally, I prefer the last half of the era, from the 1860s onward to the turn of the 20th century.
Technologically speaking, I believe that you absolutley must have three things: steam, brass and airships. Add all the rest of it as you like, but these must be included. Of course, I throw in a good many extras. I like steamtanks, for one thing. And large walkers. And automatons. Science runs amok, with mechanickal devices and even a new threat on Venus which I am slowly developing. This may even be an original idea that I have fabricated. However, I have spoken of it elsewhere and am considering using it as a professionally produced setting, so I don't want to elaborate any further upon it at this time.
Geographically, the skies are certainly not the limit. Many of the VSF gamers I know are heavily influenced by the groundbreaking roleplaying game Space 1889. This brings in the idea of the luminiferous aether (a not totally discredited though quite unpopular theory, it turns out), and interplanetary travel with steam powered ships. Colonial imperialism spreads to Mars and Venus. Another strong influence has been Edgar Rice Burroughs's novels, especially regarding Barsoom.
Politically, it seems a very straightforward European-dominated genre. Few players are without British troops, and Germans seem to follow immediately behind. One thing I do find interesting is the profusion of alternate nations. My own Republic of Texas, for example, or Fenian Republics, the Empire of California, and so on. Another common variance to our actual history is in the American Civil War. It is frequently either ongoing or a reversal of outcome. Other real nations are sometimes brought in as well. France, the Ottoman Turks, Russia and even Japan are not uncommonly encountered in many VSF settings. And then there are the Martian native states as well, usually modeled somewhat on the Princely States of the Raj.
Well, those are my thoughts on the matter. As you can see, I have a very broad scope for my ideas IMVSF. Consider that I am also working on four different scales, and the enormity of my VSF projects is dimly glimpsed. Even I am not sure what will eventually be included, but I do know that I have not yet begun to paint all the figures, vehicles and terrain that I need to see it to completion.
Let me know what you think.