The next time you are trying to add some flavor to an encounter, consider toying with the area lighting. Certain PCs may have difficulty discerning targets off in the distance under dim light conditions (PHB 262), granting those monsters a slight advantage (-2 penalty for attack rolls). Having the complete absence of light can really make an encounter difficult as the penalty for total concealment bumps up to -5 (PHB 281). Keep in mind though that area of effect and close attacks don’t grant a penalty under these conditions.
Counter this with monsters that have darkvision. There are a surprising number of those that do, even at the heroic tier such as undead-types. If you throw in some manner of an environmental hazard that simply reduces (or removes) the player’s light source against some creatures with darkvision, you suddenly have a very challenging encounter.
Having a fight in an open underground chasm, with a constant wind reducing open torches to dim light can make things interesting. Consider having magical effects also. A zone-like effect that nullifies light to utter blackness might make a fight against undead a memorable one.
I’d use something like this sparingly, as it might make certain players upset about being unfairly put at a disadvantage. At the same time however, it might make a few players shine by employing non-combat oriented skills and abilities (say the wizard casting a simple light cantrip).