The one thing that absolutely changes how combat is run in modern games is the presence of firearms. Melee combat implies a certain level of…connection, which makes taking someone completely unawares difficult. Once you’re talking about high-velocity rifle rounds, this is no longer the case.
Distance of engagement is a big deal because the dynamics of starting an encounter change. Although you can play the sneaky character and get the drop on someone with a dagger, taking someone by surprise becomes a matter of planning once you have a rifle of some sort, especially is we’re assuming relatively modern technology (which means accurate modern firearms). For the GM this is both a blessing and a curse, because killing someone in the abstract becomes easier.
Of course, killing someone is easier because your weapons are more lethal. D&D is a game that’s built around wearing armor. Armor helps in modern settings, but tends to be cumbersome and can only do so much. This tends to mean that combat is fast in modern games…whereas a D&D combat I ran could take 6 to 8 rounds for evenly matched opponents, that same matchup in Cyberpunk 2020 would take one round, and you wouldn’t even make it through the whole turn order. This also means that, if you’re a GM who isn’t terribly mean to their players, ambushes and the like have to be done carefully. A sneak attack involving a sniper bullet could easily kill a character and give him no recourse, while a fusillade of automatic gunfire can, in the right circumstances, be a TPK without even trying very hard.
What makes this truly interesting is that the law gets involved. My earlier post on law enforcement spoke to how the ever-present threat of being arrested will suppress your party’s illegal activities quite significantly. Well, this goes doubly for carrying weapons openly (unless your setting is like that). In places where guns and armor are restricted (which is quite a lot of them in the modern world), whole new sets of emphasis exist for determining who will win a fight. In my groups, it ends up being about hiding the guns. In other groups, it could focus on martial arts, or stealth, or even intimidating people into looking the other way. Whatever mechanic works for your group and your world.
There’s not a whole lot of delving to be done on guns. Your combat planning changes completely, and the environment changes completely. There are ways to change the effect, but that is most likely a post for another time.