Remember the real estate maxim; Location, Location, Location? The mamxim I believe in GIS is Specialize, Specialize, Specialize.
Specialize in a specific industry. Knowing GIS is not enough you need to have specific industry knowledge too. WIth proficiency of the tools and specific industry knowledge you will know what is good data and what is not. Only intimate industry knowledge will provide the insights to offering your services.
To Furter my point, how many SAP consultants have superior experience with particular modules (i.e. Financial, Logistics, ). You would be surprised to find not many. This is the crux of the problem in consultancy.