In No One Cares About Your Cool Game Idea, Mike Birkhead, Senior Game Designer at Vigil Games, said:
Ideas are worthless. The only currency that holds any weight is the ability and drive to execute. That awesome game idea you have, the one that’s going to “change everything”, the one that you’re going to sell for a million dollars, the one that no one has come up with yet… frankly, no one gives a shit. Harsh, but then, the truth is not pleasant; it is just that, the truth. [my emphasis]
Copying ideas and features is okay. (Copying code or textures is not, unless it is open source and the copying is compliant with the license.)
Notch himself copied the idea of the final boss fight. Years ago, before many Minecraft players were born, someone named Scorpia used to write a column for the magazine
The game industry has been copying itself for years! The software industry, too. Have you ever used a spreadsheet? I bet it wasn't Dan Bricklin's VisiCalc.
Many of the cases so far of mods implementing each other's features are the result of parallel development, not copying.
We've been in the early, "wild west" phase of mod development, where it was frequently possible to write the first mod to implement a particular feature. As the Minecraft mod scene matures and it becomes harder to achieve a "first", we will increasingly see mods incorporating each other's features.
In cases where parallel development isn't involved, when implementing a feature from another mod the polite modder will acknowledge the mod that served as inspiration.
But above all, no modder should criticize another modder for copying, or imply a lack of creativity. Copying is merely a sign that the "wild west" phase of Minecraft modding is coming to a close.