Consultants are companies that learn how industry leaders do things, and then sell that knowledge to the laggards, in effect. Sometimes they will even help implement the solution, for a fee of course. Consultants' primary assets are their knowledge base, people, and reputation - all non-capital assets that don't really show up on a balance sheet. While most of these firms are technology based (such as Diamond Management & Technology Consultants (DTPI)), many others advise companies on how to find and hire executives or implement businesses processes like accounting or human resources.
Moat building for Consultants is somewhat similar to employment firms, in that reputation is an important factor. However, it was mentioned above that reputation is just one of the primary assets. Knowledge base is an important asset, and consulting firms with a wide breadth or depth of knowledge certainly have a leg up on the competition. For example, Accenture (ACN) has consultants in a wide variety of fields, from IT to accounting to human resources. Large, global firms are more likely to use a single consultant for these problems than to divvy up the work among 3 different ones.