That's an interesting thread, thanks for the link Mr. Man. I sympathize with the frustration that the developers of TDSM and TShock must feel, having limited ability to change the game experience for players and admins. I feel that the core Terraria engine is awesome, and could be used flexibly to create a wide variety of 2D gaming experiences, were an API available to code against for client and server mods. This would extend the life and breadth of the game significantly. Even if they doubled or tripled the content in 1.1, the re-playability of the game is still limited without having an open-ended plugin system. This is a no-brainer.
One feature I'd love to see is the ability to control player inventory (even if consent of the player was needed.) I'd love to be able to create a series of jumping puzzles, like a classic platform game. In my current world, I used Tedit to create a giant underground labyrinth with a prize at the end. However, the possibilities are limited by the fact that most players have gravitation potions and grappling hooks for climbing, and you can't limit this. In other words, there is no possibility of one-way access. Another limitation is the lack of tunnels that can intersect without being accessible to each other (foreground and background tunnels.)