It's true that in my tradition there is nothing a deacon may do that a lay person may not. But it's also true that even priests and bishops spend the vast majority of their time doing things a competent lay person could do. I think it's a mistake to boil an order down to the specific tasks exclusive to that order, but rather to look at the life as a whole.
So, for example, look at the exhortation in the ordination of deacons:
"Our Lord and Saviour Jesus Christ lived and died as the servant of God.
All who follow him are called to serve God in the world,
setting forth Christ's kingdom through the power of the Spirit.
Christ has called you to the office of deacon.
You are to be an ambassador of Christ,
serving God as you serve others in Jesus' name.
Proclaim the good news of God's love,
so that many may be moved to faith and repentance,
and hearts be opened to do justice,
love mercy, and walk humbly in the presence of God.
Let the transforming love of Jesus be known to all
among whom you live and work.
Strengthen the faithful, teach the young,
search out the careless and indifferent.
Encourage the members of Christ's body by word and example,
ministering among the sick, the needy and all who are oppressed or in trouble.
Together with your bishop, priest and people,
you are to take your place in public worship,
assist in the administration of the sacraments,
and play your part in the life and councils of the Church.
You are to preach the word of God in the place to which you are licensed,
and to pray and work for peace and justice in the world.
As a deacon, you are to model your life according to the word of God.
Study the Scriptures, reflecting with God's people upon their meaning,
that all may be equipped to live out God's truth in the world.
Put away all that does not make for holiness of life.
Be faithful in prayer, that you may have strength to run the race that is set before you."
And then the service goes on with the examination.
But that is a portrait of a deacon. And even though it has been nearly five years since those words were said to me at my ordination, I can't type them without tears in my eyes. "Christ has called you..." and this is what you are to be.
It is more specific than a baptismal vocation, which can be served in any number of ways and contexts.