Here are the only possible reasons I can think of why a front wheel would not be reversible:
--asymmetrical wheel due to space constraints (folding bike or recumbent bike)
--disc brake on one side
--wheel designed for one-sided attachment (that is, mono fork)
Ball bearings roll both directions. Spokes go both directions. The rim brake surface is the same in both directions.
That said, just like with symmetrical hubs and symmetrical rims, there is no earthly reason besides aesthetics, why a slick tire like a Continental 4000 should go one direction or another! The minimal tread pattern on those tires is for looks and it doesn't matter which direction it turns. And believe me, the rubber itself doesn't care which way you go.