First, its nice knowing that each judge on the show brings in a unique background, varied experiences and industry influences to the judging panel. Having a working knowledge on many generes is key, as the whole panel would be expected to have a knowledgeable opinion and to speak intelligibly about everything from hip hop to the waltz.
Given this buffet of differences, it would come as no surprise that each would have a stylistic choice of judging when it comes to evaluating a contestant.
Craig seems to be the judge that consistently focuses on the tree. Was there a small section where the choreography let the celeb down? Were the extensions in the arms and legs not freer and extended enough? Did the celeb stumble on the lifts? If so, the whole dance is to be discredited. Disheartening, but true, especially outside of Strictly.
Len on the other hand focuses on the forest. Despite the dance foibles, does this celeb have the raw materials? Having seen both latin and ballroom, is this contestant in it for the long haul? They developed the swing/sway proficiently at this expected time frame? Nice progress.
Definitely not black and white in categorizing their judging styles (as there are moments when they do overlap) and there is certainly nothing wrong with each of their approach. But when comparing these two judges even further, two intangible qualities (imo) come to mind that separates Len from Craig as respective judges.
Willingness to Admit Mistakes
What's interesting to watch, is when Len goes on a long tirade (complete either with a red face, posture up, finger pointing, etc) denigrating either the celeb/pro, only to come back the next week (or two) to make a 180 reversal in his critique (if it's warranted based on celeb/pro correction) and a full transparent admission (sometimes with adlibs) of his past indiscretion or newly revised feedback. Admittedly, all the judges have moments when they point out moments of past inconsistencies of their own doing (and credit should be given individually), but the intensity with which it's carried out by Len both home and abroad is interesting. Many may be quick to point out Len's willingness to admit his mistakes as a sign of weakness or ineptness on his part as a judge; on the contrary, it speaks largely about his open-mindedness and flexibility, and reinforces and dignifies his qualification as a judge even more. Very few people have the mettle to admit mistakes, especially in the face of a roaring public and a divided panel.
Ability to Read Dancers Effectively
Len has the uncanny ability to identify high potential dancers, a feel that can only be honed and refined through years of experience, reflection, application and thoughtful thinking. The beginning of Series 11 of Strictly had Natalie Gumede hailed by Craig as the “front runner” of the competition. It seemed true, as Natalie demonstrated promising technique (for the first week) and seemed as an obvious candidate who would outperform her peers. However, Len was quick to point out that this wasn't the case, as there was another contestant whose skills have yet to be refined, who also had a larger catalytic learning capability and who could take it to the next level. He identified Abbey Clancey's week 1 waltz as “the dance of the night.” Not so much that Len identified a potential winner, but rather through the mental constructs with which he arrived to this decision, it enabled him to read someone effectively and accurately – a trait that few possess.
While Craig is accomplished and occupies a niche on the panel, I would have to respectfully disagree and point out that Len has the breath and depth of knowledge and experience, to provide an accurate assessment of a couple's true worth, abilities and potential, when it comes to judging contestants on this dance competition show.