A bad consultant: Isolates the problem, goes away, solves it alone and brings back a solution to a flowing, ebbing and changing organisation that does not need an 8 week old solution.
A good consultant: Embeds herself in the organisation, understands the heartbeat of every facet of the people, processes and technology. Creates a space for change by asking the right questions. She supports with domain knowledge where needed to create sustainability in the team. A team she will one day step back from.
She knows she has done a good job when she can walk away and leave an organisation that is stronger, more resilient, more self aware and ready to tackle the next set of problems or opportunities.