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?
I faced that problem myself. We had many strategical stuff that we wouldn't reveal too investors to early because most of them were possibly involved in the sector. Therefore we wanted to avoid any risk of leaks that could be used by potential competitors.
So I used a technique I could name layered project pitching or even incremental project pitching.
The first time we met an investor, we discussed the first version of the project. Depending on his reaction & questions, we would go deeper into the layers, and so on until he is convinced to go with us.
It worked since we got much more investors that we needed without having to reveal our biggest secrets stuff in the lower layers, that in our sense would make our project a success.
I think that investors are less interested by the project than the entrepreneurs that will drive it. However a serious investor will want to know why you will succeed.
So my advice is don't go into the details unless you have to.
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.