The dark is unfortunately (or fortunately) not very dark, but sometimes switching between a human and a elf character makes a visible difference. In combat, as Ecc2a stated, attacking an enemy in a dark area might give you a disadvantage (you role two dice and take the lower result). You can avoid that by either canceling the disadvantage out through giving yourself an advantage (high ground for example) or placing a light source next to the enemy. I might be wrong but placing a light cantrip on your melee's weapon should help - or having them use a weapon that is illuminated to begin with - same as all other sources of magical or non-magical light. I think throwing candles around is also an option or those cute little hearth light bombs.
Along with your own primary stats being low, you can also miss often because you try to use an ability with a spell save DC against an enemy who is especially good in that stat. That is, casting a spell with a CON Save against a character with a lot of constitution might not work out so well.
