If antibodies are produced against other blood groups' red blood cells, why can't antibodies form against white blood cells, of any blood group? (even the same one, as MHC will be different in almost all individuals)
Antibodies and other immune responses do indeed occur against the donor's white blood cells. However, white blood cells are present in relatively tiny quantities (3-10 million WBCs per ml of blood vs 4-6 billion cells per ml).
Because the number of WBCs is so low, the immune reaction to them is also proportionally lower, with fever being a typical (and minor) complication of the reaction to WBCs. Some blood banks remove WBCs from blood to prevent these reactions.
For more information on the possible types of reactions to blood transfusions, see Wikipedia.
EDIT: Even in cases where the transfusion does not produce any symptoms, an immune reaction is most assuredly happening. In those cases, the immune system is simply killing off the foreign WBCs without any fuss.
Adding - very late to this - the most common symptom of a WBC incompatibility in a transfusion is a short-lived rash (looks like "heat rash") and low fever.
The HLA antigens are not very "antigenic" compared to the main blood group antigens, and the problem is mostly seen in people who have had multiple transfusions over a period of time.