I'd say motivation, interest and making use of the opportunities to practice. More specifically, my problem is usually verbs rather than nouns, because if you can speak to some degree in a language you can describe what you want to say. Sometimes people say "Aha I got it! you mean ..... don't you?" or they do not bother to tell you the word, just nod to tell you they understood. However, getting a grasp of how those people's mind work is the hardest&most useful thing. For example, English speaking begginers in foreign languages usually start their sentences like they do in English, and even if they know the right vocabulary the listener can be confused. This usually happens when the syntax of the language you learn is different than your native language. Not only syntax, but also word usage. I sent an Uyghur person who speaks Turkey Turkish a very sad song, and he told me "This song holds one's heart!" I understood his meaning when he sent a crying emoticon, obviously because I don't directly associate sadness with a feeling which holds one's heart. So, grasping the logic (or that there's no logic) behind phrases and word usage is another important element.

first, you've really got to have desire, enthusiasm and dedication in learning new language. then, find a partner whom can teach that particular language you are learning. :)

горячее желание овладеть им и постоянство занятий

For my part, I would say that it is the range of vocabulary you possess. I think so because for me it is the most critical thing to know how something is called. I'd rather say something grammatically incorrect but using the right words because, in my view, it makes it easier for the opponent to understand you. Nevertheless, I must add that learning grammar is of paramount importance as well, but for me it is second important.