What exactly would you do differently working full-time that you can't do now while still at your job?

I ask because I thought the same thing when I first considered quitting my full-time engineering job to freelance. But then I came to the conclusion that it was just a justification that would allow me to immediately do what I wanted (quit) while putting off the tedious part (find more freelance work).

In the end, I chose to be a bit more prudent, and actually line up enough work to comfortable quit without worrying too much. So I went out and found enough work that would allow me to make as much as I was making at my full-time job for at least 3 months (noting that my rate was significantly higher, so it only amounted to finding about 25hr/wk of work for 12 weeks). Even then, I was still nervous as hell when it came time to give my 2-weeks notice.

By far the easiest way I found to line up a solid 3-months of freelance work before I was actually doing things full-time, was to find a full-time development firm and work out a subcontracting agreement with them.