A Consultant Who Can 'Do' The Work Themselves
The ultimate consultant is one who can not only provide you advice, but also make it happen. A consultant who regularly works 'hands on' with his products will have greater product knowledge and be able to create solutions to your specific problems more readily.
Good Communication
A good consultant will also be a good communicator. Communication skills are important for consultants as they need to help you as a business person achieve your goals; this means solving certain challenges, designing solutions and working to ensure a generally favourable outcome.
Someone Who Achieves Your Goals
You shouldn't need to communicate all of your business goals to your consultant. Your consultant should be able to suggest ways you can meet your business goals without being prompted. Your consultant should also be able to give you advice on changing tasks you have set so that the outcome of the task is more accurately aligned with your business objectives and meeting customer needs.
Good Product Knowledge / Solutions Architect
A consultant with good product knowledge (not just of their own products but also their competitors) is someone who will provide you with more objective advice and can help you make decisions more quickly.