@Thai1on
I always enjoy a good discussion.
I do want to point out that, so far, pretty much every post I've made in this thread has been confined to what I'll term "Real World History". Jules Vern, Bergius Process, Coal Mining Industrial History. I haven't ventured into (Or even anywhere near) the "What if" of a Steampunk world till this post.
Before one gets too far into trying to build Cultural or Economic factors, it usually makes the most sense to me to have a think about the beginnings of whatever story you end up wanting to create. History is what shapes peoples experiences, and thereby the decisions they're likely to make. You can leave this blank and just pick random elements you like, but this can be a tricky balancing act. It usually works out better to have some kind of cohesive idea about where things came from, so that you're less likely to randomly introduce things that seem out of character for the story 1/2 way through it (Can be a total Story killer, like introducing a random new never before seen character as the perpetrator in the last chapter of a Murder Mystery. This has been done, but is rarely done well).
Along that line of thought, there seem to be 2 major avenues of "Beginnings" most successful Steam Punk stories seem to take.
The first is to leave the creation of the world a blank ambiguous never explained mystery. These kinds of stories tend to be more wildly random (Which often times makes them more fun to read, as long as the story never gets too detail oriented), and instead just borrow bits and pieces from our "Real World" (Coal, Steam Power, Physics, English, Victorian Era English Culture, Air Ships, Just to name the bigger themes generally). ALOT of Steam Punk Short Stories and Novella's use this method (If you're only writing 5-50 pages of a story, you probably don't want to spend too much time on back story).
The Second, more difficult approach, though I find thoroughly more satisfying, are the ones that take Real World history into account, but twist and edit it starting at a certain point. They'll usually pick a certain point, either in the entirety of Human History (Things like "What if Nazi Germany had won WW2 and this delayed the exploration of Oil Resources", or the "Earth never had much oil to begin with", as you've suggested), or my personal favorite, merely changing small things in a wide variety of ways (Such as, another suggestion of yours, Coal was just too plentiful to ignore).
The reasons that caused the development of the society you're trying to write about will play a huge role in whatever technology or culture they end up exhibiting. Steam Punk is perhaps the first genre of fiction where the Props can as often as not play an even larger role then the Characters, as such it makes sense to spend alot of time crafting their world in addition to the characters, though ultimately they will both affect each other. Steam punk just wouldn't be steam punk with out certain elements, its what defines the genre as separate from say, the works of Alexandre Dumas or Jane Austin.
All in all, as I said, I prefer picking a divergent point, and this can be almost anything. Ill use the Boneshaker as an example, where the setting was early 20th Century Seattle (Which I found hilarious, living in Seattle, and the author even got the landmarks correct, though she futzed with bits of the history, which was entirely acceptable as the result was such a great story), and through the course of the invention, theft, and misuse of a giant steam powered tunneling machine, a Zombie making gas is there by released from the bowels of the earth. The divergent point is the use of Steam Power in such a crazy scheme as this, and the result is a rip-roaring wild adventure with Zombies, Steam-powered Airships, daring characters, and all set in a Walled up deteriorating zombified Seattle. It sets the tone for a whole series of books, of which I've heard the Author has continued to work on.
Bringing this back into the world of Steam Trains, if you say, made a diverging point from history in the proliferation of Coal, then one can assume that there would be differences in the speed at which humans progressed through our cultivation of Steam Driven technology (Imagine if the Dark Ages has been an industrial revolution with the introduction of steam instead?). Further, the first major uses of oil may not have been the internal combustion engine at all, but perhaps plastics, or some other later discovery, say Solar Panels for instance. Part of why oil, diesel, and petrol development happened the way it did was entirely because a Steam locomotive consumes so many resources, and has but one path to follow, it has been long viewed as highly inefficient by the engineering field (not to mention the accountants) of our world. There are many reasons for this, and I only have at best partial data, but when one of the most efficient designs ever produced can eat several thousand pounds of coal an hour and hundreds or thousands of gallons of water, without ridiculously easy access to those resource, many will ask the question "Is there in fact not a better way"?
If the proliferation of coal was so prevalent, more efficient ways to acquire it surely could have followed much quicker, higher efficiency could have been attained sooner, to the point where oil may never have developed the prominent position it now enjoys in daily life if for no other reason then transportation. Nuclear technology (Which is essentially a Steam engine with perhaps the most efficient fuel source currently harnasable by man) may easily have developed along different tracts to become a more common source of transportation energy.
The places you can go with this kind of thinking are innumerable. But, they require a firm grounding in at least some basic history, and a basic understanding of Sociology (The study of groups of humans, the trends, beliefs, preferences, thought processes, and actions there-of) helps tons as well. Economies and Cultures are alot easier to read once you have some ideas about how sociology works, as long as you can acquire starting data to work with.
Regardless, the diverging point I just explored would pretty much require a world where coal was so prolific, you could pretty much trip over the stuff just walking out your door. It has to be so easy to attain (At least at the beginning of the divergence), that people have to ask themselves "What can we do to get rid of this stuff?".
You could go other directions, such as no oil, although thats a much more difficult proposition since coal itself comes from similar processes as oil geologically speaking, and itself can be made into an oil or a lubricant. Not to mention machinery requiring some kind of lubricant as well. Either way, the more you learn about the topic you're trying to write about, the more believable and interesting it will become, Promise, and this is especially true in a genre as varied, crazy, and in-animate object centric as Steam Punk can Be.
Anyway, food for thought,
Falcus