I am slowly experimenting with the icon bar, and finding it to be very useful.
I've found one icon/spell that is not working on the icon bar - Summon Familiar - even though I did a Lookup and it appears to be right.
I have my hotbars arranged in a 2x4 grid (recently moved two of them off to the side). The icon bar is stretched clear across both columns and I have made it as short as possible vertically, however, there is a large gap below the icons on the icon bar that I cannot get rid of.
If I knew what to look for, I'd modify the .xml myself...