This was where it really went wrong. People would continue meetings, get on the phone, sneak off for a smoke, go to their car and take an early lunch and the warden was left to search them out. ...
On a plane, the "what to do after you're out" instruction is usually a simple "and move away from the plane". No idea of how far (personally I'd get as far away as possible!), no idea of what to do once you are "away". (Would it do any good to say something in the briefing, probably not).
This is typical of evacuations. The cabin crew follow their procedures on board the aircraft pretty well. It's once the passengers are off that things become "muddled".
Now, it would be great, if all of the passengers marched in step to a safe location, formed up in ranks, and sat down so that they can be counted - in reality that's never going to happen.

The comment above was about a practice - but in a "real" event, some will want to walk about to "sticky beak" the fire, several will now want to film the event on phones and cameras (perhaps to sell to the news/post on their f@cebook page etc

) Many will want to call friends and family, or run about looking for same.
If the event occurs on an airport, they will also be unprepared. A recent evac. at my company, no transport was found, passengers spread out - some walked to the terminal. The terminal staff refused to let them in the doors (most of their documents were, of course, left on the aircraft) as without documents they were trained to keep the terminal sterile.
When some passengers finally got into the terminal, some of those local to the area were so fed up they went home! The airport had no plans to provide a room to gather the people together.
Fire staff wouldn't allow anyone back to the aircraft after it was safe in order to collect their baggage. Eventually the crew organised a bus back to the aircraft which they filled with the carry on baggage, to return to the terminal etc etc.
Does any of this matter? No.

It is all a hassle, but it is all "safe" once the people are off the aircraft! It would be nice if it were done better, but it is such a rare event, and involves so many different agencies that I doubt it will ever get any better!