Ability to attach effects (scrip-fu plugins, filters) to layers. When you change
the layer the chained effects (the final outcome) should be recalculated.

The feature would support experimenting crafting of images and reduce the amount
of work with the images. Now if you for instance start building a nice image
with something say crafted on wood it takes 5-10 layers and all kinds of tricks
to reproduce the perfect image. Then you notice something that went wrong
earlier (ie. typed "Gimp ocks!") you have to go the phases again. 

With dynamic layers all you would have to do is to slightly touch one layer and
voila. The amount of useless work would in time decrease greatly with the
feature. It isn't in the same league with the scripting since scripting is
pretty much for only the nerds and not for real human beings.

The absence of this feature is a real productivity problem at least for me and
because of it I rather am paying 800eur for the *cough* licences.. It's imho a
real blocker and the software is like from the 80s on this field atm.