I signed up at New Year and am really enjoying working through the Podcasts. They are a fantastic resource, but I think that the learning centre would really benefit from being focused around a spaced repetition flashcard system, perhaps like the one over a "reviewingthekanji".
I'm sure that you are all aware of the popularity of SRS programs such as Anki, iFlash and Mnemosyne just now for learning japanese (perhaps in part stemming from the great advice over at alljapaneseallthetime.com). They are excellent language learning aids.
One of the problems of using these programs though, is the constant sourcing and inputting of sentences for review. JapanesePod101 has a fantastic collection of material - really, some of the best available - and it would be really great if, after finishing a lesson, I could select which sentences or vocab I were important to me and have them copied over to my SRS deck in the learning centre for review. Currently I have to cut, paste, format, check, and sometimes retype your content when transferring to my external SRS (It's a pain)- and I'm sure that there are many others who do the same.
This would have a great benefit to me, really streamlining my study sessions and would surely encourage people to spend more time and interact on the site, since an SRS system requires constant daily review. Again, look at "reviewingthekanji" for an example of the popularity of the system and the community it has spawned.
At the moment (and this is just my personal opinion) the learning centre is a bit disjointed. There are a few fun distractions, but nothing that encourages rigourous daily visits. And there is nothing that gives the sense of growing achievement that focusing the learning centre around a good and prominant SRS system could bring.
Sorry if the above sounds negative. It's not meant to be - I just feel that there is a great opportunity here to "take things to the next level" or "bring it all together" (to quote a couple of Peterisms).