In vanilla Destiny, there were usually three difficulty settings: normal, hard, and impossible. All three were dictated by your current level, and while it wasn’t advised that you do the impossible or hard difficulty, the game would still let you try your best. Within reason; they wouldn’t let a level 8 Guardian shoot for a mission meant for a level 24, that’d just be stupid, but ultimately none of it really meant anything.
With The Taken King, the hard difficulty has been completely wiped out and there’s only normal and impossible difficulty. The latter is dictated completely by the amount of Light you have, and all missions currently have a Light requirement of 240 before you can do them on impossible difficulty. You get Light by getting better gear, but it could be a while before you reach that 240. And if there’s a higher difficulty afterwards is up in the air, as well. It would’ve made much more sense to keep the hard difficulty around, because if you’re ridiculously overpowered for some missions, you may wonder why you’re even doing them.