"Custom practice" option
Currently, there is no effective way to manually select and practice the words that are the most difficult, and thus need the most practice. I have a suggestion.
Often, when I'm browsing the vocabulary section, I'll see certain words I simply don't know. Currently, there is no way to make use of DuoLingo's tools to learn these words. One can only hover over them, and try to memorize each word on its own. Otherwise, one would have to wait until the word gets registered as needing to be practiced, so that it would be put into a vocabulary test.
It would be awesome to have a checkbox beside each word in the vocabulary list, which could be ticked. Then, it would be possible to select "Custom practice," and a practice-session with all the selected words would be presented to the user.
I feel this would give users a simple and effective way to practice exactly the words that they judge themselves to be bad at, thus effectively improving their language skill.
This is unlikely to happen. Duolingo's devs are working very hard to try and figure out how to automatically determine these words and work them into the practice. I've seen some pretty massive improvements since I started with duolingo a couple months ago. Just keep using "practice weaker words" they use some pretty interesting techniques!
"Practice weakest words" does help a lot. Nevertheless, it misses certain words that clearly need training.
Coming from a software background, I estimate that this improvement would not be very technically difficult to implement, at least compared to an optimization of 'weak-word-practice'. At the same time, I personally feel it would be an incredibly useful tool in becoming proficient with a language. So the trade-off, to me, seems worth it.