They're looking for a repeatable, scalable business model that they can pour cash into. Beyond that, it's all a matter of the investors' risk tolerance. (How little proof do they need before parting with their money?)
The metrics you're looking for are just pieces of the puzzle - you need to tell a story that says "if you put money into our black box, you'll get more out". There are no magic numbers.
As far as I can tell, "traction" is a synonym for "the VCs I play golf with have heard of you" -- social proof and all that. But I'd also love to hear opinions from people with direct experience here.
In addition to the already stated metrics of #users, #payingusers, #visitors, and %conversionrate, I'd hope VCs would look at $revenue, $profit, and %monthlygrowth.
