What counts as the "really secret stuff" which shouldn't be shared with potential investors?

After reading significant opinions on NDAs and how they might apply (see this article for example), I'm wondering, what counts as the "really secret stuff"? What's reasonable to keep a secret from investors, especially in the context of web-based businesses - where there isn't anything that's significantly new or original in terms of technology?

My sense is that you should only talk to investors you are willing to trust and tell them everything. It is hard enough to get the attention of an experienced, reputable investor not to have to try to pitch them without the full impact of your proposal. If you are dealing with someone worthwhile, they will not have built a strong reputation by stealing ideas from entrepreneurs. If you waste their time with a meeting in which you are only willing to talk in generalities and withold what you consider to be the essential elements of your plan, they will almost certainly pass and you will not even have given it your best shot.