As part of my job I get to work with some really smart business people - both clients and fellow employees.  They come in all shapes and sizes but at the top of the pile, they have some common characteristics that I try to take note of:

Ability to Pick Out the Important Stuff - They are drawn to the important things even when they aren't on the page or spoken.  They apply small items to larger goals and bigger pictures.  This allows the important stuff to display itself to them. 

Questioning to Draw Out Strengths (or Flaws) - They question to understand and test ideas rather than to display their superior knowledge.

Efficiency - Efficiency in writing, efficiency in speech, efficiency in hearing.  Not a lot of rambling.  There is power in efficiency.  My father ran his own company (I considered him to be very smart) and I was always amazed at how precisely he could get at a subject at work and home.

Speaking to Be Understood - Rarely do I notice them speaking in jargon.  Complex issues are explained in clear terms so that they are better understood.

Interest in People - Most of the really smart people I have observed are genuinely interested in people.  Their employees, their customers, people with whom they interact.

A Natural Curiosity to Understand - Alongside good questions, they have a natural curiosity to understand.  Financial structures, marketing plans, solutions, ideas.  They are generally not dismissive but curious.  

I always come away from the interactions with these people knowing that I've learned something.