Compo - a few ideas that you might be interested to hear about with regards to evacuating aircraft.
As gadgetman says, the lack of visibility is a big point. Over or under wing exits tend to have better visibility as they are windows (at least on all the aircraft I've operated on!). However, the main doors are very different. I found that when a telescopic viewer was fitted it was still difficult to have a realistic view of what was outside - even under normal conditions it is sometimes difficult to see obvious things like a GPU.
Another factor with over/underwing exits is the lack of space to move the window. I believe (and may be wrong), that after the Manchester accident, the AAIB recommended a better seat pitch around these rows, but airlines have been reluctant to introduce this.
I also feel that it would be difficult for cabin crew to control passengers around exits as there is not enough space. SOPs state that you stand by the exit and shout at the passengers to move towards you, giving a "helping hand" to anybody that hesitates at the top of the slide. Most of the aircraft I have worked on tend to have bulkheads either side of the door, which makes that particular instruction rather hard to follow! It could also be problematic as once you have opened the door, people are going to want to get out ASAP, and will not want to move back to let you back into the cabin.
If you'd like any more information then feel free to e-mail me.
[ 03 December 2001: Message edited by: Deep Cover Gecko ]