I played on a bay12 server hosted by GG at some point, the issue with public servers is I think, progress between people changes rapidly. If you don't all play at the same time and together you rapidly distance from each other, and people are going to be ready for hard mode at different times then others, and then in hard mode various events will be unlocked before others are ready. It sorta actually sucks. Building stuff together is cool though.
This is it basically.
I find MP in this worthless unless you're doing it with friends who all sync up at the same time.
A single world doesn't have enough shit to split with many players, especially if some of them just blitz through everything in 2 hours. And you know they will. They always will.
Every "public" server will have:
1. A guy bringing in massively overlevelled shit
2. People blitzing the game and unlocking all the hardmode shit so no other new player can join
3. People blitzing every single chest on the map
4. Some bugger cheating his arse off for stuff
It's not fun for anyone joining late or anyone who isn't actively doing one of the above.
It's much more fun to just MP with friends and then when you are all done, just shut down the server so everyone can be on the same field later.
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But at the same time, shit like this came from the early Bay12 servers: