First, the riders need to navigate 9 sections of cobblestones - a difficult task on road bikes, especially for the skinny types that contend in major tours. Then there's another wild card: the weather.
The latest model forecasts show a trough of low pressure draped across much of central Europe tomorrow. Belgium and northern France are in cool, showery weather during the day tomorrow. The GFS puts the heaviest precipitation a bit further to the northeast, but there's a decent chance that the cobbles will be wet during the race.
It could be breezy as well, although I don't think that will be as significant of a factor as the wet cobbles. Looks like it could be a very interesting stage.