IMO I think it was how the RAF used the Hurricane in conjunction with the Spitfire, using radar/intel/etc to try and match the Spitfire against fighters and fighter bombers when possible, and have the Hurricanes hit the slower aircraft. I suppose this can be overstated, as I'm sure fighter command sent whatever they had a/c wise to intercept whatever was needed, and couldn't always pick and choose what fought what and where, but I do know this was the preferred match up.
I think the ruggedness of the Hurricane played a major part of its success against both fighters and bombers. I've also read that the way the .303 guns were grouped closely together provided a more dense pattern of bullets at the ranges they were shooting at the bombers with the Hurricane, but again, due to the RAF's policy regarding harmonization and convergence, and that many squads apparently "did their own thing" in this regard, it's likely hard to say what kind of effect this had. I know I've seen it come up in various books and documentaries regarding the Spit's spread out armament, and the Hurricane's focused and tightly grouped guns. I personally don't know, just find it an interesting subject regarding the differences between the two primary British fighters, and an important one, since the guns are really the bottom line of any fighter when it comes right down to it, since they are what causes the enemy to go down after all the other stuff like aircraft performance and the like.
Like the OP said, I'm sure British/RAF tenacity played a role with both British fighter types vs the Luftwaffe. Fighting over "home turf" in friendly radar cover can't be overstated either IMO, and obviously had an equal impact on both Spit and Hurricane.
IMO the major factors in the Hurricane's success were its ruggedness, the more simple materials allowing easier repair and maintenance, it's ability to out turn Luftwaffe fighters that in the heat of the moment decided to turn fight with it, it's more focused firepower, a noteworthy subject due to the very limited firepower of the .303 round - every little bit makes a huge difference type of idea. IMO it was sort of the dark horse of the Battle of Britain, that one factor that ended up surprisingly punching well above its weight class versus a much vaunted opponent.