This is the big one, the premise that determines so much of the rest of Traveller. To be explicit:
1) The time quantum of interstellar travel is 168 hours +/- 10%. More energy means more distance, but does not shorten this time. Even in Miller's more recent T5, higher TL drives like Hop, Skip, etc. increase distance but the time quantum never shortens.
2) There are no ansibles, subspace radio, or any other way for energy to travel faster than light speed. Sending a message means sticking it in a starship and jumping.
This premise produces strong consequences that do not exist in much of Speculative Fiction, and might be undesirable for many potential players:
1) No interstellar "Race Against Time": The first you hear about that planetary disaster Jump-1 away, at least 1 week has past, and it will be at least 1 week before you can do anything. This blows up a slew of Star Trek and Star Wars staple plots. In Traveller, you deal with the aftermath of such events, or you can only Race Against Time at the planetary or system level.
2) Express Boats are Inevitable: I suspect that many players think of Express Boats as a part of the iconic Third Imperium setting. But in a universe where every trip is like an ocean voyage, optimizing trips and logistics seems inevitable. The rules virtually guarantee their existence.
3) Why We Have Nobles: Nobles and neofeudalism are another iconic feature of the Third Imperium. While less inevitable than Express Boats, long travel times means that systems of control require large autonomy for decision-makers, whoever they may be. And if you want your will carried out, you need trusted followers who will obey order when you aren't around.
4) Space Travel is Unpleasant and Most Never Do It: This one is so important that it will get its own entry, but I wanted to cite it here. A huge subversion of nearly every major Speculative Fiction franchise.
5) Little to No Standardization: This one you might want to debate me. You certainly don't have to run your setting this way and I suspect many do not. But it seems a very natural consequence to me. For example, at TL 7+, something like cellphones and the internet is likely to emerge. But there is no reason whatsoever that a Star Empire would standardize such tech across worlds. This is because Most Worlds are Unimportant, another upcoming entry. You land on a new world, you buy their version of a prepaid mobile, and good luck figuring out their social media!
I could go on, but I think you get the picture. What I'm interested in hearing is 1) the degree to which you accept or reject these premises, and 2) how do you address these premises or alter your setting to get around them. No judgment here: I like most of the underlying premises of Traveller, and part of the fun is thinking about how to work with them to solve problems. But if these aren't fun for you, I imagine they aren't fun for some others. Hearing how you address these issues is probably useful for others.
1 - Basically that's correct. There are some subtleties here though. Travel is still limited by time in jump, that's a given for the classic Traveller jump drive. The subtlety is a question of time / distance and jump factor. In some areas it may be weeks/months for news to reach worlds because no one is travelling to them regularly or they are off the main routes. In other instances - especially for really important messages ("The Zho have invaded the Marches!") you'll see dedicated jump-6 couriers and dispatch boats making full distance jumps and they'll be carrying the message at a rate of roughly 18 parsecs/week. So in some cases information may reach more distance locations faster than nearer - all depends on message traffic speed.
2 - Definitely a difference here than other sci-fi settings. Star Trek had it's subspace, Babylon 5 had jump space (same effect, different process). In ye olde times of radio or telegraph it took time to relay a message since you couldn't directly talk over long distances. Today it's all satellite based and digital switches and communications travel at roughly the speed of light.
However, ye old days of travel covered this rather efficiently. In the earliest days of sail you simply put your mail and cargo on the next available ship (and sometimes sent it on multiple ships if you wanted to be sure to get your message through). Later you started seeing the advent of clipper ships and their ilk who prized speed above all else. And some people were willing to pay a premium for passage and cargos, though most still went in the holds of regular cargo ships that plodded along at standard speeds. In the era of passenger liners you saw the same - most passengers and some cargo travelled in the holds of passenger liners. The lines that did nothing but carry passengers across the Atlantic regularly vied for the fastest crossing record, most luxurious, etc. And people paid for that. Mail went aboard all kinds of ships, with letters and small packages going on fast passenger ships.
If you look at other gaming universes, such as Cherryh's Merchanter universe, it's very Traveller-like. Every freighter carries messages from system to system, station to station on regular routes along with their cargoes. Private messages are encoded but general news and things like stock market data are freely transmitted upon entering the system - this includes any filed ship flight plans so that a system would have at least some knowledge of new ships arriving. Though ships on a regular route had generally known arrival windows. Governments and rich corporations have smaller courier ships that will sometimes make longer, more direct jumps. Pipers Space Viking universe also had communicatoin at the speed of ship.
3 - Nobility is not a requirement for control of an empire that has slow communication. Many empires in history were able to control empires similar to Traveller. Democracy wasn't a concept back then - having a basically hereditary ruling class was the defacto norm across the world. Depending on the empire you had essentially civilian administrators (governors) appointed by the central government who ruled in name of the empire and were defacto kings - and in some instances they let that idea go to their head and declared themselves kings.
4 - A number of disagreements here have been posted on the other thread. Space travel in the future should not be considered "unpleasant". A more descriptive adjective(s) would be "time consuming", and "expensive" - though neither is out of reach of the average citizen. It just means you can't hop aboard a starship and travel to another star system as easily and cheaply as people travel today. As we see from many nations, especially in the industrialized world, people DO take long vacations, sometimes around the world, for weeks at a time. Millions of people do that every year on this planet. So it seems entirely reasonable and within the expected realm of possibility that regular space travel in comfortable / luxurious settings will be seen. Though don't expect the average citizen to travel from Core to Regina on a regular basis. Travel within the sub-sector should be considered a norm.
5 - Standardization is, and isn't, to be expected. Nobody is going to go through all the effort of inventing an integrated circuit and build it differently from world to world. Nor will people invent a 5mm screw on one world and a 5.5mm screw on another. Just like today you see standards across the board for many, many, many things. Often that's because the industrial tools and machinery are built by a few companies and that becomes a defacto standard. Humans being humans DOES mean that we should expect some variations though. The ubiquitous 200 ton Beowolf free trader may be common, but like semi trucks today you see many variations of the tractor across the world. So the 200 ton class ship may be a norm, but it's reasonable to assume dozens of variations that can be regularly seen crossing the spaceways.
For anyone who's curious, there is a really good set of books on ships, their history, operations, and such, from Conways. It covers military and civilian, and the books cover from trimarans all the way through to supertankers and carriers.
