I have a friend who always rode mountain bikes, when he switched to a road bike the thing he struggled most with was the visual aspect of the skinny tyres/wheels, and how they reacted with the road by his movements, more of a mind thing rather than anything practical or physical....
I can agree with that completely, before I switched to road somehow looking at those thin skinny tyres was almost enough to declare road bikes insanely dangerous. My brother still asks me how I ride with tyres that thin but now it's not an issue for me at all.
In the beginning I swore I'd never stand out of the saddle because everything felt so very different and 'wobbly' on a road bike it was just extremely reactive. Having gone from a heavy dual suspension bike fondly named my "work out bike" I felt like I was cycling on a feather. It is all perspective though, eventually I just got use to it.
Although I won't lie, sometimes the weight (or lack of weight) catches me out when I go to reposition the bike and throwing it into the right position throws me along with it and that catches me out.