I'm wondering what could be done in a mod that is placed in the server only, not the client, and what couldn't?

I know that GUIs are a no for server-side, but what else?

And for example, let's say I have other mods the server and client (let's use thaumcraft as an example) and I were to hook into the thaumcraft API on the server, could I change the behavior of a block? Say, make a node with infinite aspects?

Question is too broad for you to get cover-all answer.

 

Vanilla client is designed to be able to interpret data sent from server.

Server loads/saves/computes data which is send to client to show to player, ANYTHING that client was not designed to do, well - can't be interpreted.

 

Note that you can use client's code to represent something it was not directly designed to show.

E.g: you can use vanilla GUIs to represent custom server inventories. Since client cannot interpret custom packets, you can only use vanilla ones.

 

Server mods can do pretty much anything they want, problem is that clients wont be able to display it or even crash trying.

E.g: Making custom entity in server mod would be problem since client wouldn't be able to spawn it locally (since it doesn't have it), BUT - with ASM you could edit server in a way to tell clients to spawn zombies as representation of custom entities with stats (health, speed, etc) set by server. But that still won't give you full experience sice not every entity is zombie-like (or other vanilla entity).

Hey Ernio, I had a question in similar regards to his. Does forge allow you to program server side mods that do not need to be downloaded by the community?