The title "Guide to MongoDB for Startups" is hand-wavy, and is more like a high level overview of MongoDB architecture. If you are looking for a "Guide to MongoDB for Startups", it would probably be part of a much larger series of technology choices for startups. Since I found the other lacking, I'll give you my perception of a Guide to MongoDB for Startups:

* Do you have a customer yet? If not, technology choices do not matter, go build your product as fast as possible and get a customer.

* Is your system starting to have slow performance during max usage? If so, every system will have a few easy optimizations, find those using something like NewRelic. Aside from those, technology choices do not matter, throw money at any scaling problem (at this point, it typically isn't that much money). This will optimize your time for sales and marketing to get more customers.

* Is your system starting to buckle under the weight of customers? If so, go hire someone who can scale everything you learned from your customers, and not someone who bends to the latest trends on HN.

Here is an article on my philosophy and experience with scaling businesses and architectures: http://blog.mongohq.com/changing-the-growth-formula/