The main features are that they are stronger and lighter than normal steel blades. Also, the Valyrian steel has been lost along with Valyria, meaning that there only a few Valyrian Steel blades left in the world. There are only a few people in the world that can actually create a new blade from an existing Valyrian steel blade; thus making them extremely expensive.
Valyrian steel blades are lighter, stronger, and sharper than even the
best castle-forged steel, and feature distinctive rippled patterns
similar to Damascus steel.
Only the greatest weaponsmiths can reforge swords from existing
Valyrian steel, but the secret of creating such an alloy was
apparently lost with Valyria, making those remaining weapons highly
treasured and extremely rare. The blacksmiths of Qohor claim to know
how to reforge Valyrian steel, and Tobho Mott did reforge Ice at the
request of the Lannisters, in King's Landing.