I think a good language learner is someone who can not only memorize but really take in vocabulary. This might be someone who is able to understand the roots of the word and how the word was created. It might be someone who really has a quick grasp of syntax and the subtleties of language. Learning a new language will require quick vocabulary acquisition. A good language learner will be able to pick up both new words and new grammar rules naturally.
A good language learner also recognizes root words and cognates, which can be very similar across languages. It makes it easier, especially with verbs. Finally (though this might not be a very constructive comment) I think it helps to be young. My personal experience involved learning Spanish as a youngster and learning French as an adult in graduate school, and I found it much harder the second time around.
A good language learner has to have a good ear for accents and dialect. Generally people who are good at mimicking others find that they can quickly develop a good accent in a foreign language.
A person who is good at learning languages is one who is not afraid to make mistakes. Some people don't want to try to speak a new language because they know they will say stupid things. But you cannot learn if you feel this way. You have to tell yourself that it doesn't matter if you make mistakes. A good language learner is brave in this way.
A good language learner practices A LOT. It takes a lot of time and dedication to learn a new language. Being able to practice well is another thing. It's not just vocabulary that is needed. A good language learner practices proper grammar techniques, speaking, writing, and reading. One other big factor to learning a new language that is important is they also will learn about the culture behind the language.
I was naturally good at French; however, because of that, as the difficulty of the language progressed, I grew more fearful of making mistakes when it comes to writing. Over time, I learned to get over my fears and make mistakes with confidence because there are teachers and peers out there who are willing to help me correct them. Also, I was fortunate to be friends with someone native of France, so she helped me a lot. Speaking the language out loud and listening to videos of French really helped me a lot. Practicing during my free time was also essential.
Interest in others views , talktive, fearless to speak, social , like debates, these are basic qualities in good learner.
A good language learner must have an ear and mind for the finer aspects of the language like accent, idioms ,syntax etc.The learner must also have the willingness to use the language when called for and to experiment with it boldly.if a learner has the adaptabilty to think like native speakers, it will add to the rapidity of learning the language.
Having a good ear to listen as given in an earlier post is important to get the right accents and dialects but having a good flexibility and ability to reproduce the same need excellent control of the muscles around the oral and buckle cavities. Language like German, French, Arabic, Punjabi and Pushto etc need this control to produce the correct sound.