Overview of Learner Characteristics

There is a considerable amount of research on individual characteristics of "good language learners." Factors that may or may not make a difference include:

The learner's aims and purposes in learning the language
Does the learner want to get a high score on a standardized grammar test, or is the learner's goal to be able to communicate with people in social or professional situations? Some learners aim for accuracy and others aim for communicative effectiveness.
Exposure to the language
Has the learner only been exposed to more formal language in the classroom, where there is often more emphasis on accuracy? Or has the learner been able to interact with native speakers outside the classroom where more informal varieties of the language are spoken? So-called "heritage speakers" of a language "have incomplete knowledge of a first, native language acquired in childhood" (Montrul, 2014, p. 93); they may have been exposed to the language at home, but have little mastery of formal varieties because their home language is not a medium of instruction in school.
Age
How old was the learner when he or she began to study the language? Our ability to learn a second language gradually declines as we get older, starting in early childhood. For example, Scovel (2001, pp. 113-116) and others argue that native-like pronunciation is almost always easier to achieve before puberty.
Motivation
How much does the learner want to learn the language? And is the learner's motivation more practical (instrumental motivation), or social and emotional (integrative motivation)? Although motivation type is hard to measure, many researchers and teachers believe that a learner's willingness to put in a lot of effort is a very important factor in determining success in learning a second language.
Personality
Do personality traits such as empathy (Guiora et all, 1972) and risk-taking (Beebe, 1983) affect success in learning a second language? Some research suggests that one's ability to empathize with speakers of the other language causes one to speak in more native-like ways. Such personality types as 'extrovert' (sociable, out-going, with many friends) vs. 'introvert' (quiet, observant, with smaller numbers of close friends) have not been found to affect success in second language learning. Both personality types can succeed.
Language aptitude
Do some learners have an aptitude, or special ability for language learning? Learners with a higher aptitude for formal classroom learning draw on a combination of linguistic, memory, and auditory ability.
Learning styles and strategies
Do students' preferred learning styles and strategies affect their success or even the nature of their learner language? Researched learning styles, or preferences for learning second languages, can be concrete, analytical, synthesizing, communicative or authority-based. Some learners want clear rules and others are content with ambiguity. Learning strategies may include memorization, use of flashcards, or mnemonics.

Most language teachers believe that individual learner characteristics cause students to follow very different paths to success in learning a second language. What do you think? Can you predict how learners will do in your classes, based on their individual characteristics and language background?