Introduce sentence based flashcards. After all, one knows a language when one is able to produce and understand sentences one has never heard before. It is simply not natural to learn words in isolation
See http://www.glowingfaceman.com/blog/sentence-mining/ and http://www.alljapaneseallthetime.com/blog/how-to-banish-boredom-from-sentence-mining-sentence-picking

The new HSK has 6 levels ranging from basic (150 characters) to advanced (5000 characters)

It would be very useful to have these word lists. You can find them at http://www.confuciusinstitute.qut.edu.au/study/proficiency.jsp

Reading characters is one thing, but it is much harder to write them from memory. A writing test (perhaps limited to single character phases) would be really valuable. Many sites have handwriting recognisers, such as http://www.yellowbridge.com/chinese/chinese-dictionary.php and http://de.mdbg.net/chindict/chindict.php
A lot of them seem to be very similar so perhaps it an open source project is available somewhere.
Maybe the writing test could be a seperate module rather than in mixed with the other tests.

There's lots more language out there!

not just in the ones where a character is displayed and the user needs to translate into english and pingmin. I have a feeling repeating the audio in all question types would improve retention, since I am trying to learn pronunciation as well as vocabulary..
Otherwise great product, would see myself paying for it if it expanded to phrases & grammar (and other languages)

allow users to go back and edit custom lists after they've been made (for example, to add words, pics, etc).

I already know about 175 words and 75 characters in Chinese. Let me skip to level 2 or 3 or beyond without having to wait for days!

Right now the response to voice recognition exercises is binary. Either you are given credit or not. That can be pretty frustrating when you cannot seem to get a word right and are not quite sure where you're going wrong. Can you either:

a) Put some shades of grey in between right and wrong (e.g., "almost"), or
b) Give feedback on which syllable or part of the word is not being pronounced correctly?

I am currently in Australia, therefore my "tomorrow" terms are never actually available at the start of the day. I've been trying to guess when my terms will become available, and it is sometime in the early evening.

It'd be great if users could select their timezone in their account settings, then have the terms become available at the start of the users actual day.

1-4 for normal choices, 5 for None of these and 6 for I don't know

it would be great to have an iPhone app allowing to go on with the study while away from home. Would be even better to be able to use this app offline, and then re-synchronize with the online database when reconnecting the iPhone on the network

why not use color for tones to enhance the memory remembering process

I'd like to understand if I got close or not. I'm not sure if I just got an accent wrong, one character, or the whole thing. Would be helpful if it indicated what I got right and what I got wrong.

When you mastered the word (above 70%), next tests should be automatically without pinyin. The option to switch pinyin on/off is not flexible, because if switches off pinyin in new words also.

running 1440x900 resolution. my browser is windowed to be a little bit smaller, but normally i don't have a problem seeing things anywhere else.

Sometimes the audio pronunciation is either missing or is incorrect for some of the terms. You might consider adding a bug button that alerts the administrators more quickly that there is a problem with a term.

Currently, the system continues to move forward at a set pace on all questions. This is probably okay on matching questions, but on user input questions that the user answered incorrectly, it does not allow the user to compare the difference between their answer and the correct answer.

Allow students to test their knowledge without getting to go back to the term if they get it wrong. Also there would be no pausing available so that students wouldn't be able to cheat.

Hey guys. You're teaching incorrect pronunciation in various words. You really need to show how certain tones that follow other specific tones changes the sound of the word. People are going to have a VERY hard time breaking the habit of saying things incorrectly.