My mouse wheel is what I prefer to bind to first. It is what I use for my main nukes and anything that is spammed. Rolling a wheel is much easier than bashing a key. I was happy to see that there was the option to bind to the mouse wheel, although it did not function correctly when I was playing surely it will be fixed. Binding to the wheel is a deal breaker for me. I know that seems silly, but I would never want to play w/o it.
I also can't imagine playing w/o Bartender and Parrot. I applaud their UI and think they did a nice job, but custom UIs are necessary now in gaming. People are too used to playing with them. It just feels like you are being denied full enjoyment.
What bindings or addons do you consider must haves? As in, you really feel are must haves.