Great pics, and link! Thanks.
Wow, subtle difference! So when the LSDs go, it's because the clutch plates have worn out? . . .
The plates themselves are not in contact with each other (at least they're not supposed to be), so normally they don't wear out. However, the plates are very close to each other and are perforated. There's a special viscous fluid between the plates. Alternating plates are attached to one side gear and to the differential case. When they turn at different speeds, which happens when one wheel spins relative to the other, the fluid between the plates heats up due to the "shearing" effect between the plates. As a result, the fluid "gels". Because the gelled fluid is not only between the plates but through the perforations, when it becomes stiff it resists the relative movement of the alternating plates.
I believe the most common failure in the V-LSD is the seals, which separate the plate area from the rest of the differential. (It's that extended "housing" to the left of the crown gear in the pics.) Once the seals go, and the viscous fluid either leaks out, or becomes mixed with the gear oil in the differential, the gelling effect is lost. Loss or mixing of fluid could happen very slowly over a long time. The seals might still cause the drag that leads to both wheels turning in the same direction when manually rotated, but the LSD won't be effective.
See also the attached description from my 2007 FSM, and http://en.wikipedia.org/wiki/Viscous_coupling_unit
, which has photos of the plates (not Subaru LSD but essentially the same idea).
(p.s., woops, see I doubled with no64terry).