Having played a lot of newbie mixes this weekend, it seems like there are some very common mistakes or misunderstandings that newer players tend to have each time, and there isn't really time to explain all those things while you're playing or right after a game. I think it would be really helpful to create articles or videos going over these things...I mean some things you just need to learn from experience, but having concepts introduced or reinforced in an article or something might help them learn it faster.
Just off the top of my head I think it would help to go over things like obvious hiding spots, when/where to expect to get flanked, capitalizing on player/health/positioning advantages, how to get picks, not having your whole team cap a point, how to not play roamer like a pocket, going to health packs instead of your medic, when and when not to peek/take damage/go for risky plays, crit heals and heal priorities, where people should generally be holding at different points, basic positioning concepts and not giving up space for free, not being overly afraid of committing/dying, when/how to use kritz, where/when you should use uber and how to create a more favorable uber exchange, what classes to focus in different situations...
I'm not the most experienced player ever and I'm definitely no Sigma of TF2 insight, but I do understand a lot of those things (regardless of my inability to actually apply them when I'm playing sometimes lol) and could maybe attempt to write out some of them, in consultation with better players. I don't have a whole lot of time though, so it would take a while.