1) What is transportation like? Are horses common or is walking on foot the norm? Or is there some other transportation to fit in with a steam punk setting? This is important because sometimes you don't want the characters to get somewhere quickly and if they have access to horseless carriages, that might be harder for you to do.
2) What do your races look like? Are they just as the book says or are there important differences? In my world, elves with red hair have a special legacy, they are descended from Veo Sunstar, and only that line can bare elves with red hair. Things like this are not only fun to put in, but gives your world depth.
3) What groups can your characters inspire to be part of? In my world there is the Wolf Warriors and the Society of the King. In Forgotten Realms, there is the Harpers. Little groups like this help give depth to your world and give your players some goal to reach.