What is required to build a good mod?

Imagination, persistence, programing knowledge...

...also novelty, "replayability," and seamless integration of new features. Aspects of the mod should seem entirely new and surprising to the player. The whole package must have the right balance of challenge and gratification, so that a person continues to feel satisfied the second time they experience the mod. And the third. And the fourth. There cannot always be something new to discover, but what added content there is should not start to feel stale after a month or two. For the latter of the things I listed, just about any modder can slip in new systems, stations, weapons, and ships. That's a given. And yet... I have seen things on many mods that my gut recognizes immediately as out of place - as wannabe Freelancer content masquerading in disguise. New features should be noticeable, but also subtle enough not to spoil the spirit of the game or represent a negative shift in quality (i.e. it bothers me when added tradelanes don't line up at junctions like the ones in vanilla, when difficulty of gameplay spikes unexpectedly upon jumping into a mod-added system, if added content leads to unbalanced combat... or when I dock at a station to discover, for example, that the Liberty Police that own the base are actually Junkers and Xenos in the bar). The little differences like that set a "cool" mod apart from an excellent one.

Patience and regular backups

Oh, and the storyline has to make you do one epic thing (like blowing up smth HUUUGE, or jump from one airplane to another or smth)

Everything made original, Beautiful enviroment/background, Effects, Determination, Common sense, Friends ;), Computer with over 1.0GHz Processor, and atleast 32MB of video memory

PS: and I kinda like putting some secret stuff in and also some lovely huge weapons >: D