Definitely transaction fees.

For mass adoption, it is unconceivable to have transaction fees. We've shifted to a zero fee world. Even card are zero fees now...
In a user experience perspective, transaction fees belong to the past and are a real drag on the adoption.


On the other hand, how could Bitcoin survive without fees when the 21m will be reached? (from a miner perspective)

I am very curious to see how Bitcoin will behave that very day.