How long should a pitch deck be to keep the investor's interest?

How many slides should an ideal pitch deck have?

What are your thoughts on having some subtle humor to keep the investor's attention span in check?

I'm not much of an expert in this, so somebody else will probably come along with a better answer. (That's OK.)

My understanding of this is that your presentation to investors should be fairly short--about 10 or 15 minutes. That number was surprisingly short to me, considering what's at stake, but the presentation is not meant to be comprehensive, but rather a starting point for the dicussion.

Most people generally say you should have no more than one slide per minute of the presentation, so that would be about 10 or 15 slides.

The slides that you do have should contain little text. You shouldn't be reading the slides, but using them to show visual information that isn't conveyed very well with your voice. Pictures, charts, and diagrams are great for the slides on a presentation.

Nothing is wrong with humor, especially if that's a part of the culture you're trying to build. (If that's the case, and an investor hates your humor and rejects you based on that alone, you didn't want them anyway.) But I'd just add that if the business concept isn't keeping their attention, you've got bigger problems and the humor itself won't save you.

Rbwhitaker is right about keeping it short and to the point.

Here's what you should have in it:

1. Problem: describe the pain that you’re alleviating.
2. Solution: explain how you alleviate this pain.
3. Business Model: how you will make money.
   
4. Underlying Magic (optional): describe any innovative new technology behind your product.
   
5. Marketing and Sales: how you will reach your customers.
   
6. Competition: complete view of the competitive landscape.
   
7. Founders: Describe the key players on your team and advisors.
8. Financial Projections and Key Metrics: 3 year forecast containing not only dollars but also key metrics, such as number of customers and conversion rate.
   
9. Current Traction: save this for last because if you have some good traction, the last slide will get investors pulling out their checkbooks.