The difference between 7k and 5k is actually not too big, from what I've seen, like you said, itemization is one of the key differences, and also decision making. 5k players have most of the knowledge they need to get to 7k, its just that they don't use it, nor do they actually focus on getting there. The ones that do, they get there.
I think the major key is that 5k is still 'solo-minded' while 7k players have more teamplay
Oh, yes that too! 7ks know their role in the team, like for example a 5k dude that plays a core in a one core lineup will try to counter the enemies strongest hero in a 2-3 core lineup thinking its his job, when he could just get tanky so he becomes the problem. Things like that and just thinking its gotta be them that win the game loses games sometimes