What you carry with you. Which could be a lot.
Also, the starter quest would obviously limit you to what your quest allow you to make, so instead of starting in the world naked you would start with your plant fiber clothing, Stone axe, campfire, etc...
But yeh keeping your inventory is doable. It's just a player file... All tfp would have to do on their end is allow a "use player file from another save" option, which quite frankly, is a good idea anyway, and one I thought they were going with when they added the persistent profile option for servers.
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Perfect.