He was extremely smart, charismatic, and a bit arrogant (but a well-founded arrogance). From other comments it sounds like he was pleasant to young people at Cycorp. I think his peers found him more annoying.
His great accomplishments were having a multi-decade vision of how to build an AI and actually keeping the vision alive for so long. You have to be charismatic and convincing to do that.
In the mid-80s I took his thesis and tried to implement AM on a more modern framework, but the thesis lacked so many details about how it worked that I was unable to even get started implementing anything.
BTW, if there are any historians out there I have a copy of Lenat's thesis with some extra pages including emailed messages from his thesis advisors (Minsky, McCarthy, et al) commenting on his work. I also have a number of AI papers from the early 1980s that might not be generally available.