The boundary between the solid lithosphere and the partially melted/ductile asthenosphere is predominantly a thermal boundary. There can be significant compositional differences between the crust and the underlying mantle, depending on tectonic setting (i.e. oceanic crust is basically made of melted shallow mantle, while continental crust is chemically different in a number of ways). (Hofmann, 1988)
If, for example, you are talking about the boundary between the mantle and a subducted slab, the slab itself is essentially scraped flat by the overlying plate as it is subducted - but this is the top of the slab, not the bottom. (There are often heterogeneities in the structure along the fault itself, due to differences in lithology (rock type), structure of the individual plates, etc.) (Sage et al, 2006)
One thing that is important to understand about the mantle is that it is actually only about 10% melted. Think of it as a big sponge, with pockets and conduits of melt surrounded by crystalline solid. The main difference between the mantle and the crust is that the crust is cool enough that the rock is brittle - it can fracture and is resistant to strain. When the crust fractures it forms a fault along which earthquakes can occur. Meanwhile, because the mantle is hot and deep enough, the rock undergoes ductile deformation - it bends under stress - so earthquake fault ruptures do not occur (though seismic waves still travel through the mantle). (Hammond & Humphreys, 2000)
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