0) A fast main road course like any other fast main road course: straight, flat and a non-zero amount of traffic.
1) Decent surface for most of it
2) Fast turn
3) A finish just slightly downhill from the start
4) A small hill near the western end of the course that flips the wind up over it
5) Trees lining most of the course that prevent it from coming down again
5a) The course's west/east orientation means that the wind does make it back down onto it a little bit at the northern side (i.e. when it's a tailwind) but you get the full shelter of the trees for the headwind leg. Basically, every day is a good day.
6) There were loads of events run on there so everyone got plenty of goes at setting a fast time, as opposed to Levens (reputedly faster on a good day but slow as a dog on a bad one) where the number of good riders riding in good form on good days was miniscule.
7) It was fairly central and easily accessible for most of the population (again unlike Levens) so most of the good riders could get there.
The positive feedback effect where because it was where all the good riders were regularly turning up to race all the other good riders, it became the place to do a time, and the place where all the good people brought their A game.
The shelter was really crucial. I once dropped my chain near the turn at the eastern end, so after I'd remounted it I pootled back and was in a position to notice things beyond my own burning thighs. The first five miles to the top of the hill had a slightly noticeable headwind; the last mile after the hill was a case of "F*)# my life I'm being blown off the bike!"