1) you can set your output geometry being - nurbs, polygons, subDs and bezier.

2) one thing I could imagine is you somehow didn´t extrude the whole geometry but only a part of it. when extruding the complete geoemtry a bottom plane is automatically created to close the mesh. if you extrude just a single or some faces there is no bottom cause this would "double-close" the object, which means you´d get a non-manifold geometry. (more than 3 faces per edge).