Ideas about language acquisition or second language acquisition are many, but of the factors you list, I would choose routine and patterns. Having an aptitude for languages helps a great deal just because it would be easier as does the age of the learner who is immersed in the second language, but I believe that routine use of the second language and the patterns which are established lead to the true acquisition of another language. The role of the first language can be important as the addition of another romance language such as French when one already speaks Spanish; however, the role of English if you wish to add Spanish can be an obstacle because the base origins are so different. If routine and patterns of another language are established early and corrections made as the vocabulary grows create a language speaker rather than one who sort of tries. To be able to speak as well as a good native speaker is the goal, and each language has a pattern to the structure. If the learner can internalize the pattern and use the routines established early with new word acquisition and sentence structure, another language can be truly acquired.