LeBron was the new guy and there was confusion over who's team it was and who would lead them during crunch time. LeBron folded and deferred to Wade.

Miami had just finished destroying the Chicago Bulls in the round before and looked unstoppable.

After the Heat lost the series Wade had a heart to heart with LeBron and basically said "this is your team big dog, i'll follow" The Heat were a much better team from that point on.

It could be a similar problem with Steph and Durant like you said.

I definitely think the key for them winning the title is getting transition points so they don't have to get in the half court and think about who's team it is or have to deal with the physicality of a half court game.