When I probed and tried to understand what makes the work they do interesting (or not), is primarily two simple things. 

Truth: Whatever is the bigger picture, the vision, that your company is going after, you need to believe in that. The vision that you're going after and the efforts being taken towards it in terms of the kind of products being built, the kind of contracts being taken upon or the kind of customers being solicited must be aligned. You must feel that the actions of today are true to the larger vision of why the company exists. The vision itself can be anything. To be the most valuable or to have the largest market cap or to make the highest profits or to touch the most lives. It doesn't matter what it is as long as the actions of today are aligned with that. Of course, the specifics of the vision might be necessary to attract the right kind of people, but not necessary to keep them feeling interested in their work. 

Surprise: There is always room for surprise. You know your company well. You know what is accepted, what is forbidden and what is frowned upon and what is encouraged. You know what to expect as a consequence for any action that you take. The first step in surprising someone is to get them to expect something. Then over-deliver. The first step is already taken care of. All that's needed is for the occasional over-delivery. This can be anything from a higher bonus to a bigger responsibility. 

Those who found their work interesting had both these aspects going their way while those who didn't had at least one of these missing. There are CEOs who find their work interesting and there are CEOs who don't. There are teachers who find their work interesting and there are teachers who don't. Software Engineers. Consultants. Lawyers. Doctors. People who work for Google. People who work for Goldman Sachs. 

It is neither the nature of work nor the compensation nor the work-life balance that makes your work interesting. It is a combination of truth and surprise.

@HR folks, take note.

[Hat-tip to Seth Godin]