HOMO/LUMO and charge distribution
You need both an empty LUMO for the electrophile with an in phase HOMO for the nucleophile at the site of attack AND the proper charge distributions.

The LUMOs are critical; you can't shove electron density into more electron density, and LUMOs represent a lack of electron density.  However, the electron density for a nucleophile will be attracted to the partial positive charges.  

Organic chemistry normally requires you make judgments about the relative impacts of various phenomena, and this is another example.