You must a few things I find essential:
- Determination
- Skill
- Can take both constructive and de-constructive criticism (depending on what you make you may get both)
- A strong idea
Remember: mods aren't something you make over-night, they take time and effort. All of the great ones like Jenka's Nightmare can take years and many, if not most mods, run the risk of abandonment because of a lack of motivation or just you're not knowledgeable enough and your ideas become too outlandish for your skill level.
It's about finding the right amount of passion too, you can't just blaze through some progress and call it a day, you need patience. And look at other mods, see what ideas they had, try and either build upon them or make something new out of them. A personal example is in my current mod, ACSS. In it, I use a teleportation system with a "wire" hanging from the ceiling and the blocks move you from teleporter to teleporter. In WTF Story, the first level has segments separated by "wires" that act as teleporters to grant access to the next part of the level.
If you will forget what I said, remember at least this:
Patience is key.