I have to agree with the steady hand. Without it, there are no tools that will make a craftsman out of you. Most people take years to develop the ability to cut and lay off with fluid straight lines in an efficient manner. I was no exception.
Aside from that, I find that no tool is indispensable, though I have been able to make do with "makeshift" at times.
One of the first sayings that was drilled into me when I started in the paint business was;
"It's a poor craftsman who blames his tools"