What are the personality factors that are useful for effective language learning?

A typically-successful language learner displays low inhibition and attempts to be an extrovert and a risk taker. In a sense, a successful language learner has a low Affective Filter (Krashen) and is willing to experiment with the language either on his/her own (Piaget) and/or in collaboration with peers and the teacher (Vygotsky). A successful learner also has high levels of motivation including intrinsic and extrinsic motivation (see Dorniyei). 

For more insights, you may want to review "Lessons from Good Language Learners" by Carol Griffiths. 

It depends on situation and socio cultural backgroud of the the learner. Some important personality factors, that can help learn accurately with speed, are: self confidence, willingness to communicate, minimum level of anxiety, risk taking and empathy. Empathy is to live in the language you are learning.