In the modern understanding, the superficially unoccupied p-orbital on a (meta)stable carbene is not, in fact, fully empty.  Instead, the carbene Lewis structures are in resonance with dative bonds toward adjacent lone-pair or pi-bond orbitals.
Early workers attributed the stability of Arduengo carbenes to the bulky N-adamantyl substituents, which prevent the carbene from dimerising. But replacement of the N-adamantyl groups with methyl groups also affords 1,3,4,5-tetramethylimidazol-2‑ylidene (MeImC:), a thermodynamically stable unhindered NHC.
In 1995, Arduengo's group obtained a carbene derivative of dihydroimidazol-2-ylidene, proving that stability did not arise from the aromaticity of the conjugated imidazole backbone.  The following year, the first acyclic persistent carbene demonstrated that stability did not even require a cyclic backbone. Unhindered derivatives of the hydrogenated and acyclic carbenes dimerized, suggesting that MeImC: might be exceptional, rather than paradigmatic.  But the behavior of the acyclic carbenes offered a tantalizing clue to the stabilization mechanism. 
Unlike the cyclic derivatives, acyclic carbenes are flexible and bonds to the carbenic atom admit rotation. But bond rotation in the compound appeared hindered, suggesting a double bond character that would place the positive charge on adjacent nitrogen atoms while preserving the octet rule. Indeed, most persistent carbenes are stabilized by two flanking nitrogen centers.  The outliers include an aminothiocarbene and an aminooxycarbene, which use other heteroatoms, and room-temperature-stable bis(diisopropylamino)cyclopropenylidene, in which the carbene atom is connected to two carbon atoms in a three-member, aromatic, cyclopropenylidene ring.