Still not convinced. There must be 1000s of files on there now so in order to access most of them you'll need to upload 100 files each.
How many people have 100 unique files? Most people might have 1 or 2 at the most. What will happen is people will start to upload any old junk, or download files and then re-upload them, just to get more credits.
There are so many people using this forum that you only need each person to upload 1 or 2 unique files for it to work. And as an incentive they should maybe get unlimited downloads back, or at least a much higher ratio (1-500, say).
I'm not saying this cos I'm tight with uploads. I uploaded about 30 over the last 2 days - all that I have - and now I don't have any way of getting new credit to download any new stuff. I feel somewhat cheated.
All that said, still a good idea.