What makes Bob Ross such a great artist? What attributes and skills does he have that makes him so?

Ross helped to popularize a technique called "wet on wet".  That is where you put down some background paint and then paint over with other colors, objects and details while it is still wet. 

While on his show it looks like he is drawing a picture off the top of his head, in reality he sat down and sketched out each picture in advanced.  He laid out what it would look like, where each object would go, what colors he would use, etc.  He also had  to determine how to lay out a design that he could paint in the limited time he had for his TV show.  And then he painted each picture in advance.  Often make design changes to simplify the painting so he could do it quicker. 

When he got ready to film, he had the finished painting (plus sketches, etc.) setting just off camera where he could see them.  And he "copied" the picture live for the camera.   So there are (at least) two copies of every painting that he made.  His master and the duplicate copy he made in front of the camera.  (His painting current sell for several thousand dollars each.  He produced over 400 paintings on his show.)

He kept his painting very simple.  Usually landscapes with limited details so it did not have to spend three hours getting someone nose to like in a portrait.  Trees that he could make with a five or six strokes of his brush and leaves that he could quickly dab on them.  He only did one painting on TV with a person in it, and that was a tiny black image in the midground of the picture of a person in front of a campfire.

He did a lot of putting multiple colors on his brush so that he could do one dap and three colors in a leaf to give it depth.

He used large brushes to wash the back of his painting with color before he began to place objects on it.

All of them technique that any good artist should have in their bag of tirck.  

Bob Ross wasn't considered to be a great artist by any art appreciation standards, but what he did was inspire "regular people" to give it a try, using his techniques. The ones he was pleased with, he signed, but he didn't sign them all. 

So, what is art? Expressing your spirit in such a way that it sparks the spirit in someone else? I'd say so. 

Bob Ross himself said his work was not fine art. Bob Ross's show was about what it was titled, The Joy of Painting. Bob Ross help countless people how to enjoy and produce paintings they could hang on their walls or give as gifts, I do both. Bob Ross often told his viewers they should change things up, use colors they like more if they want, and to take the time at home to add more to the painting if they want, he was only showing people techniques to use. Wet on wet painting is nothing new, it has been around a long time, Bob Ross brought it to the fore to teach people that they too could enjoy painting.

Yet another 'famous' painter I have not heard of.

His bushy hair did a lot of the drying of wet paint.

“Great artist”? Not sure about that.  He does what he does very well, for a purpose ie, for tv.  I personally wouldn’t buy one.  Not my taste.

Many will argue that he is merely a great painter, not a great "Artist". Even though I respect him a lot there is much to that.

His techniques.