First of all, I would consider making an ability a component, so that it can be attached to any GameObject:

abstract class Ability : Component {
    public abstract byte Id { get; }

    public abstract Execute();
}

By making the byte id a property, you can easily override the property in each class and give it a unique value, without having to worry about keeping a list:

class MyConcreteAbility : Ability {
    public override byte Id {
        get { return 8; } // Byte value of your choosing
    }

    public override void Execute() {
        // Your code here
    }
}

(Just make sure not to use the same value twice.)

Additionally, I would add an overloaded version of the method Execute to the abstract Ability class, to serve as a message handler:

public void Execute(byte id) {
    if(Id == id) {
        execute();
    }
}

The ability's Execute method can then easily be invoked on all objects it is attached to by calling:

BroadcastMessage("Execute", 8)

Or on a specific object and all its children by calling:

gameObject.SendMessage("Execute", 8)

Of course, you can also simply call it directly.

First of all, I would consider making an ability a component, so that it can be attached to any GameObject:

abstract class Ability : Component {
    public abstract byte Id { get; }

    public abstract Execute();
}

By making the byte id a property, you can easily override the property in each class and give it a unique value, without having to worry about keeping a list:

class MyConcreteAbility : Ability {
    public override byte Id {
        get { return 8; } // Byte value of your choosing
    }

    public override void Execute() {
        // Your code here
    }
}

(Just make sure not to use the same value twice.)

Additionally, I would add an overloaded version of the method Execute to the abstract Ability class, to serve as a message handler:

public void Execute(byte id) {
    if(Id == id) {
        Execute();
    }
}

The ability's Execute method can then easily be invoked on all objects it is attached to by calling:

BroadcastMessage("Execute", 8)

Or on a specific object and all its children by calling:

gameObject.SendMessage("Execute", 8)

Of course, you can also simply call it directly.