Ooo, I like a lot of the above suggestions.
Namely, metrics. That would help us identify where we are weak and focus on those areas. SRS rank is a good one, number times wrong, last reviewed, review frequency, word source (so imported, from what lesson, from what news paper article, etc)
Tagging would be great to help with the play lists. But, related to this, I think that more list functionality would be better. So the able to click words and add to a pre existing list, or remove them. Ability to review more than One list at a time, ability to chose how the list is displayed (so begin with pinyin, or simplified, or traditional, or JUST AUDIO! Audio review would super kick ass!)
Honestly, I would like to have the ability to focus more on grammar. Initially, this could be done by reviewing whole sentences from various lessons. This can be done just like the vocab now: sometimes you get simplified, sometimes pinyin, sometimes fill in the blank, sometimes just audio & select correct character, sometimes translate the sentence.
Eventually, it would be nice to take a whole sentence and say things like, 真 is the adjective here. Which other adjective could also be used, and select from options. Or, here are a bunch of hanzi: 手，以，可，用，掏，啊, tap on them to add them to the sentence in the correct order. How about having 4 sentences that look similar, but there is some kind of grammar mistake, but we have to find the one with out the mistake.
Finally, here is the big one: selecting the correct response. So a sentence from a lesson is displayed (or played audibly ideally). You must either type or select the correct response to that sentence.
The big problem that these suggestions are looking to avoid is that I don't feel like pop-up is teaching me how to manipulate the language well. There are lots of ways to remember words, but very few ways to practice putting them all together...
Thanks for being so diligent in asking for suggestions. It makes me feel like you care. It really makes me feel like you care when I see them implemented. 泡泡中文加油！