I think it would be a great thing if we somehow could see how difficult a word or character is considered to be while training.
This could be based on how well you have done with this particular word before, or on how common the word is, or even better: both.
Like:
pinyin: hǎo
Meaning: Good, well, very
Difficulty: Easy - 90% correct,very common
The word of course is 好, the 90% means that 90% of the time, you have gotten this word correct. The very uncommon obviously means that this is a very common character.
Or another example
pinyin:dá
Meaning: The flight of a dragon
Difficulty: Impossible! - 0% correct, extremely uncommon
This time the character is 龖, which is a character thats hardly ever seen, and therefor considered very difficult.
This would make it a lot more fun to train.