Hunter's Mark is a spell, not a class feature, and Rangers can cast only a paltry number of spells per day.25. a ranger with a bow does d8+dex damage. at level 2 he can cast hunter's mark, increasing the damage to d8+dex+d6. at level 3 the colossus slayer ability increases damage to 2d8+dex+d6. with this capability, why do people think the ranger is underpowered?
It's also a Concentration spell, which means that (a) any Ranger who gets into combat is likely to lose it within a round or two, and (b) if they've got it running, they can't cast any other Concentration spell without losing it.
And a ridiculous proportion of the Ranger's spell list is made up of Concentration spells.