Research for University on Games Modification and Popularity Thread

Hi there! First let me introduce myself,
I am currently studying Games Design at University and for an assignment I have to write later on, we were asked to choose a particular theme and research it.

So here’s the thing, my theme is Game Modification and Popularity. I am looking at trends in the popularity of game mods and how the community creates standards for game modifications (and basically decides what is ‘good’ and what is ‘bad’).

So my questions are this: What makes a good mod? Are there are any particular things you look for in mods? Does it vary from game to game? Can you think of a particularly good (or a spectacularly crappy) mod that you have played/with??

Any comments are greatly appreciated (and just to clarify (though I will ask permission first) some of these answers may be recorded so that I may use them later on).