In order to support the child's development of language, trust, and sociability, fully functioning senses of sight and hearing in particular are critical. Children who can't hear language being used are at a great disadvantage in acquiring language for themselves. Children who can't see what's going on around them are severely handicapped in forming understandings of their surroundings and how to go about interacting with their environment.
I would say that another important aspect is learning to get along with other people. As children grow older, in particular, it is very important for them to be exposed to lots of other people, espeically in situations where the rules are not clear. This allows them to learn how to deal with different personalities and with conflict.
I agree about language, and another important aspect is trust. If our early relationships damage our trust in people, it is quite difficult to develop that trust later on. Trust is nurtured by meeting the needs of the developing child and by providing consistency so that the child can make a connection between expectations and results. When our needs are not met as children and we don't know what to expect, our trust is diminished, and we cannot properly form relationships with others.
I think that language is one of the most crucial aspects of early development. Even before we realize it, children are trying to understand what we say. Many people don't realize how early language acquisition begins. These early years are very foundational.