Shields[]
No ship's shields. No personal shields. The closest thing you'll find are electromagnetic fields used to diffuse background radiation.
Stealth[]
There is no stealth in space. Space is a vacuum so any heat shows up like a flare in the night sky to sensors.
FTL Communication[]
Instead of having an instant connection to the galaxy wide web, each colony and habitat maintain their own network. Courier ships passing through sell data dumps from their travels in order to keep current with the state of the Orion Arm. This also means that military order only move as fast as the courier ship. A futuristic Pony Express.
Contragravity[]
There is no artificial gravity, hover cars, or antigrav harnesses. Instead gravity is produced either via spin or acceleration. Most ships are equipped with small rotating personnel quarters for periods of drift and modern fusion drives can achieve well over a solid 1g of acceleration.
Long Decks[]
Most TV spaceships have the floors laid out so that down is the direction parallel to thrust, like an airplane. In reality the direction of 'down' in an accelerating starship would always be toward the engine. Meaning that an airplane shaped starship would have a multitude of short decks rather than a handful of long ones. Think of ships as flying skyscrapers.
The only exception to this are areas that get their gravity from centrifugal force. Space stations, rotating crew quarters and spinning habitats have their 'down' as the direction pointing opposite the axis of spin.
Ships that Bank & Brake on a Dime[]
Under Newtonian physics a spaceship would have no need to bank and swoop to come about on an enemy. Instead the attitudinal thrusters can spin a ship about its center of gravity. In addition it means that ships can't simply pump the breaks and stop in space. Instead they have to "flip & burn", turning your ship completely around and accelerating in the opposite direction.
Blasters[]
Rayguns, Blasters, Personal Lasers, and Small Arms scale Coilguns. It turns out the firearm is a marvel of engineering. The cartridge carries all of it's own energy in chemical form. The brass carries the heat away. The projectile imparts kinetic energy no matter the weather or ambient energy. A properly designed gun, like the glock or AK 47, can operate without fail for decades. The future won't bring new weapons, it will make old ones better.