Most obvious differences are cross bikes will have larger clearances to allow for wider / knobbly tyres, and will be fitted with cantilever brakes so they don't get clogged up with mud. I think they may also have a slightly higher bottom bracket to assist clearance, and as standard will be fitted with smaller chainrings as you want lower gears riding cross.
I wouldn't want one as my main road bike but think they would make good winter / commuting bikes as they tend to be fairly hardy.
Aodan has a Planet X Uncle John cross bike. He races a lot of cross but uses his for everything I think, including racing at Palace (with slicks obviously
), so if you're good enough they don't have to hold you back.