It's all of the above: better organisation at the holding gate, trickle boarding where possible or feasible, restrict carry on baggage, and SLF need to switch their brains on.
If only I had a quid for every time it's commented upon, or I see it for myself, this inability with grasping the concept numerical sequence for seating. If you get on at the front of the plane and your seat is 62F, what makes you think it's near the front??
As I get near the plane, I have already extracted my book, iPod, headphones and boarding card, dump them in the seat, bag in locker, sweep stuff off seat into pocket and sit down. Not hard. If LH, once we are up, up and away, get bag back, take out amenities and more comfortable kit, go the loo and change. Put the bag back and enjoy the flight. Equally, not hard.
People standing in the aisles faffing around, blocking every else's progress should ideally be shot and their carcases thrown onto the tarmac as a warning to others sat in their holding lounges.