One of the things I liked about BG was that it faithfully implemented all kinds of loopholes in the AD&D rules (which have since been "balanced" more-or-less out of existence). E.g. the optimal way to create magic users was as two-classed (indeed the female character you pick up is designed to be turned into a two-classed mage). So you go to level 5, say, as a warrior, then switch to mage (starting at level 1 all over again, but with warrior hit points).