1. There is strength in numbers
The Volturi are respected and feared partly because they are so numerous. Vampires can tend to be solitary creatures but sometimes band together in covens for support and security. The Volturi has arguably the largest and strongest coven around. In fact, the Volturi began to feel threatened by Carlisle's coven mostly because it had grown so large recently. They thought Carlisle might be planning to challenge them.
Besides their core members, the Volturi have many followers as can be seen in this image:
So it makes sense that, since vampire tend to live either alone or in smaller groups, most vampires respect and/or fear the Volturi and their large following.
2. They possess many powerful special abilities
In addition to their numbers, the many special abilities possessed by the Volturi also lead many to fear and respect them.
- Aro and Sulpicia both have Tactile telepathy
Marcus has the ability to recognize the types of relationships between people and their strong and weak spots.
Didyme had the power to release an aura of happiness, which could bring joy to herself and everyone around her, freeing people from sadness and making them adore her.
Volturi Guard powers:
- Chelsea has relationship manipulation
- Jane can create illusions of intense pain
- Alec has the power of sensory deprivation
Even the one's who don't a special power are still very menacing, for example:
Caius is very ambitious and can easily be considered the cruelest of all three leaders, and is seen as never extending mercy to anyone.