I've spent a lot of time studying languages. The sad thing is I've spent much of that time studying languages incorrectly -or inefficiently, which is just as bad. Whether from poor instruction or my own stubborn insistence on sticking to poor study methods I've wasted hours upon hours on language tasks that I won't remember or would be useless if I remembered them anyway.
Wasting time is common in all language learning tasks but especially when it comes to vocabulary, where the question is not only how to learn, but what. The Oxford English Dictionary (OED) contains around 650,000 entries and the average college grad comes loaded with about 60,000 vocabulary items in his head, much of which is stored away never to be used again. Admittedly, the greater part of the OED is archaic but the point stands that the greater lexicon of any language is obsolete or uncommon. The problem is that not all learning materials make strong attempts to present the most practical set of vocabulary. For example, one textbook I used to learn Japanese presented "inferiority complex" well before I could confidently order food at a restaurant or talk about the weather.
Finding out which materials are going to give the best preparation for authentic conversation or reading can be tricky: to a non-native speaker it is not always clear which words are worth learning as concepts common in one culture are often obscure or obsolete in others. Publishers are fortunately becoming increasingly aware of this problem and are trying more and more to tailor textbooks, dialogues, and vocabulary lists to drill students in versatile vocabulary.
In some languages this is easier than others. The Japanese government has long published a set of "common use characters" that provide an outline for gainful vocabulary study and a number of publishers have used this framework to create materials that quickly prepare students for everyday tasks as well as the nationally administered Japanese Language Proficiency Test. These materials range from the excellent flash cards produced by White Rabbit Press to a wide array of online vocabulary-building sites. As far as having an official vocab list goes Japanese is a rarity. Chinese, although it uses a similar set of characters for reading and writing, does not have an analogue to Japan's "common use characters," but sinologists have made efforts to identify and categorize the most important characters from a variety of authentic source material and for most other languages a similar process is in progress or completed.
At any stage in the language learning process it can be difficult to discern where your priorities should be. When it comes to concentrated vocabulary study this is especially true. So the next time you're thinking of picking up a new textbook or cramming your head with vocab from the latest-and-greatest online aid make sure you're getting what you need or you'll find your head full of unnecessary baggage that you won't use and will soon forget.
In other words, read the introduction, the about page, or what have you, and make sure the people who made what you're using are every bit as serious as you about finding the right words.
(Oh, and unless you absolutely have to, don't spend those hours and hours making flash cards when you could be getting your study on - in most cases, be it online or in print, someone's already done it for you.)