Props are nice, but more important for me is rules.
Rules need to be enforced, like calling hits, fps limits, etc.
But I also think other rules, like how re-spawn is managed, are very important and make all the difference between a 'pick-up game' and milsim.
For example, when re-spawn is too short, or when the hit people don't have to leave the area before they are back in, it's the same as fighting a never-ending supply of reinforcements. If that's the scenario, let's say Pacific War or Korea, it might make sense. But in general it is unrealistic.
I like more elaborate medic rules, too. It really adds to the experience when team mates need to cover the medic while he's treating the wounded.
In a game, the rules are what make or break the experience. Chess vs. Checkers.