In one of my recent BI implementations the customer chief architect, mentioned that he had visited many COE's and COC's, Gartner's center of competencies, but most of them soon turned into COM's, or Center of Mediocrity'.
The three things that might make a COE a COE can be summarized as follows:-
1. Business Participation, ownership and focus
2. BI Strategy, Standards and a 'Global enterprise BI Cookbook'
3. Keeping the name as COE and not changing it to anything else.
Business Participation is essential as any endeavor for excellence must be focused on enhancing business value. Business value mostly cannot be enhanced without a business view and guidance.
Strategy, Standards and a global methodology, as without that different teams can head on divergent paths
COE: as by a simple act of changing the name the very essence is often lost.