I came here to say what gkidwell said. A real engineer (in it to reward the passion, not just the pocketbook) would almost always find something that could be "tweaked", so I'm sure that particular word attracted the "some things could be tweaked" response.
I am one of the 31%. I just went into EE so I could come and live in US - this was 85. I have been working since 94 but am no more content now (2011) then I was back in 94.
I do think EE is an awesome field - esp digitally communications. The thing is I am not an engineer mentally and this gets to be eventually frustrating. A second factor is engineering education - I think my university could have done a better job balancing the theoretical and practical - to this day I don't have a good feel on how to find an impulse system response !!
Bottom line- don't enter a field if you don't have a passion for it
Again, everything is relative. I have friends that are legal secretaries and paralegals who work for real tyrants under extremely high stress. Most mid-level managers are frantically trying to justify their existence because they are even more vulnerable to cost-cutting than billing engineers. Anyone in the service industries has increasingly had to deal with a public which is willing to vent stress at them at every opportunity. Compared to what I am seeing around me I'm actually feeling pretty good.
This unhappiness which people are whining about is not because the managers are paying them poorly or giving them dirt jobs but because the rosy picture of engineering as a destination to aim for is no longer rosy. This is not very different from the dotcom bubble. The bubble burst when the companies found out that there are engineers in other parts if the world who would work with same passion for much less $$. So, yeah the leveling off is taking it's toll but I think it will find an equilibrium in about 5-10 years.
Why are young American engineers so unadventurous, why are they so reluctant to travel the world, work in different countries and get to be a part of the churn.
It's all about the "comfort zone".
I'd be interested in seeing the results of this survey sorted by years on the job. From my experience, the newer engineers are treated very differently and probably have higher satisfaction than those with a few decades of experience. New engineers are still getting good raises because their salaries have not yet approached the asymptotic ceiling.