Multi classing is one of the things 4th got rightInstead of multi-classing in the 3E sense, I think we may see multi-classing either supported by feats, or a system similar to hybrids in 4E, or perhaps both. Multi-classing in the 3E vein is actually limited to that edition (as far as I know), and 2E shared more in common with hybrid classes from 4E.
How do you think multi-classing will be supported?