A similar proposal to this has already been made. If I remember correctly, the method was called 'Generosity Indicator', or GI

Look at this thread:

How to stop Freeloaders

My idea about this is that, in contrast to what you are saying, the generosity of a user should not be measured by the popularity of the the files his node is sharing, but by the amount of bytes it actually did upload over the last 48 hours divided through amount of bytes download at the same time.
I see two advantages in this:

First, while your method measures the potential usefulness of a node to the network, this one measures its actual usefulness. Remember, users can still cut uploads manually or limit their upload rate to 0.5 kB/s or something like that! There are probably still more possibilities how to cheat out the measurement as it

Second, this method is easier to implemement. Your method would mean drastic alterations to the protocol, since the nodes would need to exchange information about which files are popular and which aren't. Using the other method however, nodes would just add their GI-number somewhere in their http download requests. If the sharing host doesn't know what a GI is, it'll just ignore it (hopefully).

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Then there is the question about how such a rating should be applied. You propose that it should affect the users download rate. I believe it could be less problematic if the GI would be applied like in this scenario:

Node A wants wants download file x from Node B, so it sends a http request to B. The user of node B has set the number of maximum simultaneous uploads to n. If B is providing less than n uploads to the network at the time the request from A arrives, A will get the desired file regardless of its GI.
The interesting question is what happens if B's upload slots are already taken. In that case B will compare A's GI to the GIs of the other uploading nodes. If it finds that A's GI is too low, A will get a 503. If it finds that A's GI is 5 or more points higher than the lowest GI of the other uploading hosts, B will open up an additional upload slot for a good network citizen. If it finds that A's GI exceeds the minimum GI by more than 10 points, it will even cut off the upload with the lowest GI if necessary.

I hope we will soon find the best solution.

Guido