Proportional Grid like After Effects
Make the standard grid setting in preference allow specification of height AND width. After Effects has a similar feature. Just tried to set up a proportional grid using Make Guides and illustrator almost crashed. Sometimes I don't want the Grid to be square.
Being forced to have perfectly square grid cell dimensions isn't always practical. The only way to have a grid that is proportioned to the artboard is to have a square artboard. Not cool.
Lindsey Thomas Martin commented
InDesign has this feature as well and I would find it very useful if I could duplicate in Illustrator the grid I use in ID for a publication when I am prepping illustrations for placement on pages in InDesign.