There are so many factors because being moved enough to vote is an emotional decision, not a rational one.
I think most people are more likely to vote if they see a dance they liked rubbished than they are if they see a dance they like praised to the skies. Running order matters. So it's not just the dance, it's the dance in the context of the whole programme. I think the 'it was good but it wasn't that good' feeling particularly stops you voting.
Personally, I think one key factor is expectation. Dances where people come out and are much, much better than I expected are the ones I remember most. So people who start very well are at a disadvantage for me, because it's really hard for them to surprise. If I like a couple, I'll still like them, even if they are good from the beginning, but it's harder for their dances to be thrilling.
And to be honest, I know I shouldn't feel this, but if I don't like them as people, I don't care how well they dance - there are 2 couples I don't like this year, both very good, and seeing them be good just depresses me. I wish it didn't but it does.