In broad strokes, I think I'd describe the two companies this way: Amazon innovates through optimization of new markets, while Apple innovates through creating new markets. This isn't a wholly fair description of either company, and as we know, there are exceptions in either direction. But that's the business model of the two companies at a 30,000-foot glance.
As you've hinted, probably the best way to differentiate the strategies of the two companies is on the risk/reward dimension. Amazon operates on a risk-minimization model, while Apple is risk-seeking in favor of outsized rewards. This is partially because Apple is still a hardware company (for now, at least), and it needs to keep reinventing its products to stay ahead of the commoditization of hardware.
Well said.
Even still, I think it's fair to say that Amazon primarily innovates these days through rapid and extensive iteration, especially on its consumer side.
This is the problem with people that don't believe in facts: they don't believe in the fact that this belief is bad for them.
[1] http://venturehacks.com/articles/jobs-customer-development
In other words, you can measure things after they become variations on a theme, because you know what to measure, and you can measure them against past successes and failures. They've minimized the "nova" in innovation. When something is truly new, like the first time Amazon opened a new product category or a new fulfillment center, quantitative analysis isn't as helpful. "Stuff is up 12%, and crap is down 5%." "Is that good or bad?" "I dunno."
From what I've heard though Amazon does have a very WWJBD culture. It's unclear how that will play out once he's no longer a part of the company.