Being able to make rules to protect my interests -- for example, I answer the phone by appointment only. Otherwise, people play tug of war with me.
None of this BS where companies pretend like you're really important, fly you down, pile money on you, and then completely ignore your expertise to the point of being hostile about it. I get paid for my expertise. My expertise keeps me from having to do pointless work and tackle impossible projects and it keeps my clients from wasting their money on them and we both consider it a good thing. Sometimes my expertise keeps me from having to do any work at all.
40 hours of programming a week is intense. Every company that hires people wants 40, at least. That leaves you sapped, unable to muster the will or strength to work on your own projects. Why the hell did you learn to program? If you're a good programmer, you probably learned it out of love, or to accomplish personal goals. You didn't start programming when you were 9 and spend your youth in front of the tube so that you'd make slightly more money than a mediocre programmer while you slave away your twenties and thirties. As a consultant, you have down time now and then, and not down time where you have to sit in an office and pretend to be busy. No guilt for playing Dungeon Keeper in the middle of the day.
Few excuses. It's purely a cash and carry arrangement. They come to you with the specs and required logins, data, and code, and you come back with a finished product. No one forcibly dumps higher priority work on you. No one holds you up for resources. No one tries to schedule tasks for you to keep you busy at every moment. No one can sabotage your own work except you -- and if they find a way to, you can make a rule forbidding the attack.
Vacation time is infinitely variable. You can work those dumb holidays. You can be a workaholic and never take a vacation. Or you can make less money and attend every wedding you're invited to.
Work late into the night. Work hard. Get your work done. Take it easy a few days. Rest, recover, work on your own things. Hard work pays -- for you, not for your employer.
Anything else is subject to the tragedy of the commons. Everyone needs to be policed and all activities justified because some people are lazy, or more realistically, because their experiments don't pan out or aren't really in the interest of the company. Everyone needs to work 40 hours because some people aren't cost effective to have around unless they're made to work a full 40 hours. Everyone has to answer the phone because some people are ineffective on email. Etc, etc, etc. Working a real job is a win if you're lazy, greedy, or unmotivated. If you're average, you fit right in. And if you're above average, the basic terms of employement and premise of the arrangement is against your interests.