When fundraising at the seed / angel level, what factors are important? Eg. proving traction, vision, customer references etc. What is best?

All of the factors you mentioned are important but if you asked me to rank them.

1. Traction: as it proves that you know who to build something people want.
2. Team: as it's the constant that matters most as the business evolves.
3. Vision: as it'll help tell the story as to why you have the potential to build a $100M company in 7 years.

If you want comparables, just look at the top trending startups on Angel List ( http://angel.co ) and you can see what kind of team, traction and vision they have.

The more of those factors you have, the faster and higher quality of investors you will be able to get involved.

There's always exceptions to the rules, but normally this is how it works.

P.S. No one talks about relationships, but that's probably #1. Your ability to communicate and connect with people who can help with introductions to investors is a big part of fundraising.

Traction always wins if you have it, because traction provides positive evidence to all the important questions: Can this team execute, are there any customers who want this, can you reach those customers, can you get enough money from them, etc..

Vision is B.S. at this stage, because existence or lack of vision won't be what gets you to the next stage, and it's quite possible that your vision will change between here and there.

Everyone has customer references, so that's not interesting, unless you have a high-dollar product (e.g. selling to enterprise) and you have only 1-2 customers but they give you real money and say good things -- that's considered good traction if that's your target.

You'll also need to be clear on what you'll do with the money, what you expect those actions to accomplish, and in what timeframe. In short, seed/angel is either to get profitable or (far more common) a proper Series A, so there needs to be a clear concept of what you'll do to line yourself up for a solid A.

Traction is king.

Your goal is to communicate traction over a period of time. Check out Brendan Baker's quora answers:

* http://www.quora.com/Brendan-Baker/Posts/Startups-How-to-Communicate-Traction-to-Investors
* http://www.slideshare.net/brendanbaker/how-to-communicate-traction-to-investors-with-brendan-baker-and-hiten-shah

"Traction is your story of momentum, communicated through quantified evidence of market demand for your product" Brendan Baker & Hiten Shah.

At different stages of corporate development you can convey different levels of traction. Early, it is about vision and the ability to build a team and early product. But the story needs to evolve. If you are still talking about vision after 4 months, you should go back to the drawing board.

Confidence that you will achieve your goals and return money to them is what matters most to investors. Confidence that your team can make that happen is #1. Why will you be the ones to succeed at this?

It depends. Sometimes, a strong team with a track record and the plan to work against a big problem / category is all that's needed. Other times, you need metrics and proof of concept. Regardless, it's going to be the people which wins the day IMO.