What do you consider to be the top keys to successful lead nurturing? So many are bad at it, that we should have a discussion, I think.

Jeff, it's a great question. Here are a few of my thoughts:
1) I've had to wean myself and the organizations I work with away from the idea of a linear sales funnel. Today, prospects progress towards revenue in a non-linear, non-predictable fashion, getting their questions answered in different ways and in different time frames. The buying process is more like a scavenger hunt, where different teams pick up items in different places and in a different order. Lead nurturing is not a linear process.
2) Materials for lead nurturing have to be very educational and engaging. Explicitly sales-y materials, if they ever worked, clearly don't work in the lead nurturing process. However, it is important to decide on the one or two big ideas that you want to convey, and that, if accepted, will tend to favor your solution in the prospect's mind.
3) Segment your prospects or create personas, developing materials for each buyer persona. This is also important because many decisions are not made by individuals, but by teams. In the buying process for a large account, there may be a financial buyer, an implementation or IT buyer and a business functionality buyer - each with different information needs.
4) Lead nurturing doesn't end with the purchase order. It should also include customer on-boarding and prospecting within existing customers for referrals or up-sells/cross-sells.

I'm sure there's much more to say, and I'm interested in hearing others thoughts.