If the Met is Italy, the Robert Lehman Wing is the Vatican - an internal, independent affiliated state with its own rules and logic.
Walking through the first time is an exercise in disorientation. Museums are sorted places with a definite rationale you internalize as you go along walking sedately through historical epochs and artistic periods. To wander into the Lehman Wing is to step out of one order into another. If you've been taking photos, you have to stop - no pictures. It's full of things that seem to be misplaced - Matisses, Renoirs and van Goghs separated from their fellows in the European Painting galleries, quantities of Venetian glass, tapestries, and Louis XV armchairs orphaned from the Decorative Art wings, relics and antiquities and Renaissance madonnas - and everything is mixed (and packed) together. Depending on your mood and blood sugar level, it's either energizing or enervating.
The story is that Lehman left his collection to the Met on the condition that it stayed intact and was displayed as a whole, so they built him a wing of his own - a museum within the museum. They even decorated the galleries to replicate the feel of his house on 54th street. That's clout.
Strolling through always makes me feel fancy and a little fusty, like the kind of person who might collect Majolica planters and glass paperweights and wear ballet flats with golden toes.