"You can't teach people everything they need to know. The best you can do is position them where they can find what they need to know when they need to know it."  - Seymour Papert

These are words that most e-learning designers would do well to take to heart. One of the biggest problems with so much of the e-learning I encounter is that it is simply bloated with too much content.  I think this is a result of two primary factors:

• Subject-matter experts demand that everything they know be included in the training for immediate mastery (forgetting that it probably took them 10-20 years to master that much content).
• Designers are used to using actual training materials as "documentation," relying on classroom instructors to provide the filters for what really is necessary to learn. Learners working on their own can't make that judgment.

A really useful method I've found for deciding what to include is to first be very specific in deciding what immediate performance outcomes are expected from the training.  Then using these outcomes as a strict filter, separate the content into two buckets: "Need to Know" to achieve the performance outcome and just "Nice to Know."  Most people (including your SMEs) will be astonished at how much of what is viewed as essential content really is just "nice to know."  Then build your instruction around just the "Need to Know" content.  Usually, this turns out to be an achievable goal, even though the prospect of teaching the originally-proposed content scope would have been impossible (or at least unbearable).

In some cases, if circumstances require that all content be included for other reasons, then just put the "Nice to Know" content into a structured reference area, accessible to the learner via a "I'd like to learn more" button, but don't burden the learner with seemingly irrelevant content, and by no means should you be testing on it.

My own slant on the sentiment  expressed in the quote from Papert is to remind myself that the objective of most e-learning is not to create an Expert, but rather that make someone minimally competent.  True expertise must develop over time with experience and with extended interaction with knowledgeable colleagues.