Practicing weakest Words...
Duolingo has this function but is it really what it says? It gives my weak words in the sentence which is not always what I want. I would love to have an option to practice just the weakest words (literally) and quickly recap. So If I want to remember how to say a dog I dont want to write the whole sentence such as "Lazy dog is eating meat under the sofa", You know what I mean? For the purpose of reviewing words(just words) it's not efficient in my opinion. I think it would be great if we had two options such as "practicing weakest words" and "practicing weakest words in the sentences"
It sounds logically what you want, but the point is that the words used in Duolingo are just a bare minimum of what you will ever learn I think. For example, now I just know the names of 10 animals more or less, but there are much more animals on this planet, so how can I learn these? Only to use my own vocabulary learning system. If I read a book in Spanish, I will encounter a lot of new vocabulary as well. How will I be able to understand these words? I think to write them down and learn them on my own, since there is no way to add new words to Duolingo. This makes duolingos main purpose more to learn "the soul" of the language rather than learn vocabulary.
I think your idea would make more sense if wordlists will evolve, like if you know 10 animals, then 2 animals will be added to the ever growing list of animals until almost all of the well known animals are covered. Or some other kind of evolving mechanism. However, I won't cry if your idea will come into existence, I think I will even use it until the time comes I outgrow it.
Anyway, here are some in my opinion, extremely good freeware vocabulary trainers for mac and pc:
Mac: ProVoc (it is retired, but works fine on the OSX 10.8) PC: Teach2000
Different options is always a good idea, but the whole idea with duolingo is that it is way of learning a language that is not based on learning lists of vocabulary and rules of grammar, but on getting used to sentences. So a better idea I think would be just to add more sentences.