Does the concept of employee engagement add anything over and above traditional concepts?
A few basic observations:


Typical self-report measures of engagement and job satisfaction tend to be highly correlated (I'll try to find a reference later, but I think in the r =.7 range).
As with many organisational psychology terms that bridge the scientific-practitioner divide, engagement has been operationalised in a wide range of ways.
Volume 1, Issue 1 of Industrial and Organizational Psychology has a focal article and a large set of accompanying commentaries on the meaning of engagement and its value as a concept. See this pdf of the focal article.