I've been on several tours, and I think what stands out for me is their comprehensive knowledge of their tour area/s and the ability to put history, culture and geography in context. I love how the ones I have had are so very capable in group management without seeming to manage the group.
I think they have all been really energetic, engaging and able to be flexible. In my mind they have all had different strengths, one for story-telling, one for history, two for history/context/location.
You may also find different opinions on local guides.
I (and am perhaps the only one) did not care for Herr Jung. I thought he was kind of mean and inappropriate at times by singling out tour members and treating them like school children. For instance our group had a couple of people with dicey knees so when we climbed up thru the vineyard in the rocky area between rows of vines, (not on the path) they asked whether the tour was going to continue straight ahead thru the tower or would we be backtracking. They did not want to climb extra stairs, but would if the tour was not going to return the same way. Herr Jung stated yes, we will go straight ahead, we will not come back this way. Then after the time in the tower he sent us down the way we had come and he just looked at the people who had asked him the question and laughed. I also thought he was inappropriately labile during his tour. Yes, war memories and memories of deceased spouses are often wrenching to talk about, but there were times he sobbed thru parts of his presentation that were not sad in the least. I know this is a highlight for many people and I was surprised at my reaction to him as I was so looking forward to his walking tour.
Local guides who were standouts have been: Vincent at the Louvre, Barry in Kinsale (also a RS tour guide), Sarah in Carrara, Roberto (I think??) in the cathedral in Assisi, Roberto (I think, again??) in Lucca, Robert (?? this is ridiculous, lol) as a guide in the Rijksmuseum, Annie in Volterra, really too many to name. I also, without question, have enjoyed the local folks who have led me thru truffle hunts, wine tastings, honey tastings, olive oil tasting, etc. All, without question. have been passionate about their subjects and eager to teach.