There are several errors:
1- the arrays vertices and triangles were just declared - you must allocate them with the new keyword specifying the desired size (they are built-in arrays, not Array class instances);
2- the triangles are wrong: the two triangles overlap, and have different winding orders, what makes only one triangle visible from each side;
3- you must assign the created mesh to meshFilter.mesh;
4- you're calculating uv coordinates based on mesh.vertices before assigning the vertices to mesh.vertices;
5- you must assign some material to the mesh renderer, or the triangles will be rendered in that lovely pink color;

function Start () {
var block : GameObject = new GameObject("Block");
var mesh : Mesh = new Mesh ();
// save a reference to the MeshFilter (you will need it):
var meshFilter = block.AddComponent(MeshFilter); 
block.AddComponent(MeshRenderer); 

var vertices = new Vector3[4]; // allocate the arrays with new!
var triangles = new int[6];
vertices[0] = Vector3(0,0,0); // I changed the vertices order to clockwise just
vertices[1] = Vector3(0,0,1); // to help understanding the winding order (the
vertices[2] = Vector3(1,0,1); // triangles winding order is what really matters) 
vertices[3] = Vector3(1,0,0);
triangles = [0,1,2,0,2,3];  // the triangles now are in clockwise order!
mesh.vertices = vertices;   // assign mesh.vertices and mesh.triangles...
mesh.triangles = triangles; // prior to use them in the uv calculation!
var uvs = new Vector2[mesh.vertices.length];
for (var i=0; i < uvs.Length; i++) {
  uvs[i] = Vector2(mesh.vertices[i].x, mesh.vertices[i].z);
}
mesh.uv = uvs; // assign the new uv coordinates...
meshFilter.mesh = mesh; // and assign the mesh to MeshFilter.mesh
}