Salem

Technically, the different servers should also represent different, far geographical locations you can't reach by foot.
So, wouldn't make sense if there were some differences in what you were able to gain in these servers?
What i propose is to introduce some crops and forageables that can grow just in some servers, or maybe have very reduced gains in the others. Thus favoring exporting of the local goods and the importing of the rare, costly (since you've got to consider the cost of the traveling) foreign items.

Yes, this is obviously something we have thought of. :)

In an ideal world, we would also have wanted to use slightly different map generators and perhaps even unique biomes for the various servers, but we only had so much time. Generally speaking, it's a nice thing, and therefore will have to await its turn in the greater spheres of nice things to be had. :)