The Meilenstein is the Standard, the wheel that put Lightweight on the map. It took them a while to come out with the Meilenstein Clincher, but it was only because they needed it to live up to the Standard they had set.
What makes the Meilenstein is the balance that Lightweight achieved with the wheel. It is light: 520g front, 660g rear for the sixteen front spoke, twenty rear. It is aero, 47.5mm deep, 20mm wide. It is laterally stiff: the spokes run from the rim, around the hub, back to the rim, and are epoxied into place. The rear spokes are wrapped at the crosses for even more lateral stiffness, which means more of your energy goes into moving the wheel forward. Lateral stiffness also means that both standing on the pedals and hard cornering are easier. And if you want a stiffer front wheel, you can opt for a twenty-spoke front, which bumps up the weight 20g but lots more lateral rigidity.
Lightweights feature an integrated magnet in the rim: you won't need to risk spokes, ruin wheel balance or disturb aerodynamics by putting a magnet on the spokes. There's also an integrated microchip with wheel data on it. Part of the reason is for tracking.