World of Storm and Fury:
1. In general, the highest technology that can exist in the world is what existed in the real world through the 18th Century, with one exception: There has been no development of cannon, firearms, or rockets beyond their use as fireworks and for show. The justification for this outcome is that magic has forestalled the creation of firearms. The early cannons were extremely difficult to use and very apt to blow up, so why would nations pursue the centuries of development needed to bring out 18th century firearms, when they could get similar results with a line of mages behind their battle line, acting as artillery?
2. The level of technology should be commensurate with the area. Many areas in the world will be much more primitive than this. The civilized cities will have items such as gaslamps and clocks and printing presses, while the more barbaric lands will not. Such devices would seem quite magical to many, and they would probably not be able to maintain such devices even if they had them.
3. As a result of the tech levels, keep in mind it is extremely time consuming to perform many tasks which seem quite simple in the 21st Century. Cooking a meal by fire, for example, is a minimum of a three hour enterprise. Writing a letter is time consuming, and then it has to be delivered...And not everyone can read, of course. A key point here is that characters without servants or slaves or willing friends ought to either be spending a great deal of their day with such mundane activities, or paying someone else to perform them.
4. Conversely, the spirit of this rule is what ought to be followed. For example, the piano is developed after our cutoff date. Yet, there is technological feasability for it to exist, there would be similar instruments it could be developed from, and it is not going to change the culture in a significant way. On the other hand, though hot air balloons and primitive steam engines can exist, these devices are unreliable and highly experimental--it would be unrealistic for a balloon air transport network to exist, or for someone to suddenly start building railroads. The infrastructure to support these does not exist.
5. As a follow-on to #4 above, keep in mind the timescale of the game is fairly short in duration, it takes a long time for any significant amount of time to pass. Even if a player wishes to 'fast-forward' some characters, the rest of the characters are moving at a more measured pace. Therefore, there is simply not time for significant technological breakthroughs to occur. A major development would be an improvement to an existing device. Keep in mind that industry in this world is cottage industry, not factories. Most of the world population is agrarian (or hunter in areas where farming and ranching is not possible)--historical norms would be roughly a 30-1 ratio between farmers and townsmen. Thus, information and development moves only slowly.