Stress
The stress factor in this job comes back to that point about safety. Outside of the everyday stresses of working life (getting up early, doing the same thing day after day), the main thing that you have to worry about with this job is not wrecking up the place with that giant machine you're driving.
Safe operation and maintenance of a huge machine (like a crane or freight container stacker) takes not only lots of practice, but also constant focus. You're always under pressure to be careful, but you also don't have all the time in the world. Move it, soldier.
Another concern that might arise from making this your career is the cash factor. Workers in this field don't make a ton of scratch, which, if you want to raise a family or have nice things, can be a big-time stress factor. Most industrial truck/tractor operators receive benefits including health insurance, but healthcare can still be expensive, especially if you have other dependents.
Of course, if you're planning to have a go of it in this field, you're probably a fairly down-to-earth type of person who can find happiness with a modest, middle-class lifestyle...right?