glDrawArrays vs immediate mode drawing, in a C++ plugin

let's say I'm just drawing a simple quad. glDrawArrays never seems to work properly, whereas glTexCoord2f + glVertex2f always does. Any of the Unity engine programmers have any idea why?

for example, this never works in a plugin in Unity, even if i push/pop GL_CLIENT_ALL_ATTRIB_BITS - (non-relevant code has been excluded). Sometimes it will draw a black quad, other times nothing at all:

Code (csharp):

1.     glEnableClientState( GL_TEXTURE_COORD_ARRAY );
2.     glTexCoordPointer(2, GL_FLOAT, 0, tex_coords );
3.     glEnableClientState(GL_VERTEX_ARRAY);      
4.     glVertexPointer(2, GL_FLOAT, 0, verts );
5.     glDrawArrays( GL_TRIANGLE_FAN, 0, 4 );
6.     glDisableClientState( GL_TEXTURE_COORD_ARRAY );
7.     glDisableClientState(GL_VERTEX_ARRAY);
8.  

but this works just fine:

Code (csharp):

1.     glBegin(GL_QUADS);
2.     glTexCoord2f(tex_coords[0], tex_coords[1]);
3.     glVertex2f(verts[0], verts[1]);
4.     glTexCoord2f(tex_coords[2], tex_coords[3]);
5.     glVertex2f(verts[2], verts[3]);
6.     glTexCoord2f(tex_coords[4], tex_coords[5]);
7.     glVertex2f(verts[4], verts[5]);
8.     glTexCoord2f(tex_coords[6], tex_coords[7]);
9.     glVertex2f(verts[6], verts[7]);
10.     glEnd();