No artificial gravity. Stuff that is possible like rotating habs, constant acceleration, and velcro booties sure. But that's it. No floaty cars, and spacecraft decks that face some pretend down. Down is where the engines are.
No stealth in space. You did read Atomic Rockets, right? I believe him. To a point -- there is a difference between detection and resolution and it has to be kept in mind.
You must sacrifice reaction mass on Newton's altar to have velocity. Fuel -- that which becomes energy -- is actually a small part of the picture. Re-mass, on the other hand, matters. While we are at it, there are no privileged frames of reference. While I plan to use Squadron Strike, this does rule out the use of movement modes 0 and 1. This statement makes sense if you read the Squadron Strike rules.
No ansibles. I like the whole 18th century no-faster-than-a-ship communications model. Empires became boring when everything could be referred to the colonial office back home.
There are a few inconvenient truths I will gloss over.
Getting from a planetary surface to orbit is inherently hard, especially without damaging the planet. There are some promising technologies forever on the horizon, and we have some future to work through the problems. At the moment, shuttle technologies used to transition between orbit and planetary surface will "just work" with some restrictions around g-ratings, atmospheric density, and fuel and maintenance cycles. Major interplanetary craft will not.
Now, something a lot less real world, but very useful. Squadron Strike has two shield mechanisms, Shields -- think Star Trek -- and Prismatic Globe -- think Alderson Field from Mote in God's Eye. These solve a couple of problems that end up patched in the Traveller game.
- Nukes. No amount of tin is going to stop a shaped thermonuclear shaped charge. Yes, they can shape nukes.
- How do you manage containment on that fusion reactor? Why the shield of course. A lot of they serious consequences of an adequate exhaust velocity on enough stuff to achieve a couple of gravities acceleration on something the mass of a cruiser requires serious handwavium padding to protect reality from the implications.
Finally, I am not going to go too deep into inter-planetary biological incompatibility except in cases where I want to make an exception for some particular point. Earth plants will grow with minimal (but not zero) effort, local proteins will be edible (perhaps after you add specially adapted bacteria to your intestinal flora). Invasive species will be a problem. We want worlds worth fighting over, not worlds worth nuking from orbit to get at the valuable unobtanium without interference (I'm looking at you, Pandora).
On the plus side, other worlds will be alien, and I will go though some effort where needed to develop the evolution and biology. Also, remember while humans may find a meeting of mind and spirit with aliens from another world they will not be able to breed with them; and anyone who wants to probably needs major therapy. I mean, seriously, you are more closely related to the slime growing on the inside of your toilet then you are to anyone you are going to to find on an alien planet no matter how good they are at DBA.