I feel like what's missing from this though is sexual attraction - you are not necessarily attracted to your best friend. If there's no spark, it's just a close friendship. At least to me there is a distinct difference.
I understand it like it's all on top of sexual attraction.
I totally agree with that. The parent comment contains the detailed description of, "marry your best friend."
And you can make checklist from that, to measure how "best friend" is your partner.
But wait, doesn't that mean you're already in their "friend zone"? :)
If they have a friend zone it's because they are using other criteria to select a partner.