This may not be precise enough for MO, but I'll give it a go.

Let $M$ be a symmetric closed monoidal model category with unit $u\in Ob(M)$.  We define the vertices of an object $A$ to be points $x\in \operatorname{Hom}_M(u,A)$.  We define the fibre of $f:B\to A$ over a vertex of $x$ of $A$ to be the pullback of $f$ by the map $x:u\to A$ classifying $x$.  

It can be shown in the category of simplicial sets with the cartesian monoidal structure (and unit $\Delta^0$) that a Kan fibration is acyclic if and only if all of its fibres are contractible (I'm pretty sure that a similar statement holds for CGWH spaces in the Quillen model structure).  However, the proof uses specific properties of the category of simplicial sets to prove this fact.  Is there some general additional structure on $M$ that we would need to prove this fact in more generality?