It would be nice to be able to change default tab order for container managing columns like GridLayout and FieldGroup. Currently, order is by default column by column, then line by line. For some forms it makes more sense to do line by line first.
Of course it can be overriden manually, but this is the kind of things mouse-born developers forget easily to dev & test today when they add a new field, and voila you have a messed tab order late in the development process. Often not noticed before production.