The distinction that defines Vaikuntha is structural rather than scenic, and it is easy to miss because English translates both words as heaven.
Svarga, the heaven of Indra, sits inside the cycle of rebirth. It is a reward, entered on a balance of merit, enjoyed for a period proportional to that balance, and left when the balance is spent; its residents return to birth. Puranic literature is candid about this and treats svarga as a comfortable trap, since pleasure there consumes the merit that bought it and produces no new store.
Vaikuntha is not a higher floor of the same building. It lies beyond the material nature altogether, in what Vaishnava texts call the unmanifest sky, and the soul that reaches it does not return. That non return is the technical claim, and it is what makes reaching Vaikuntha liberation rather than a good rebirth.
The descriptions follow from the claim. There is light without a source and therefore no shadow; there is no decay and no fear, since both belong to time; the residents are described as having forms like the Lord's own, four armed and richly ornamented, attending on him with Lakshmi beside him. Theologians of the Vaishnava schools distinguish several modes in which a liberated soul may be with God there, and the traditions that most emphasise devotion tend to prefer service in a distinct form over merging, on the ground that merging ends the relationship that made liberation worth having.
The name is generally read as the place where there is no obstruction, which states the same point again.