I'm a veterinarian and small business owner in California. My biggest challenge is capturing all the "stuff" throughout the day and finding time to process them all at the end of the day. A lot of things collected today need to be processed and done the next day or two. So most of the time, I can't put things off to be dealt with at the weekly review.
For instance, I see maybe 20 appointments in a day. Most of those patient exams become a project (some of them generate more than one projects). We do blood tests that will have results the following day; I'd need to call the client with the results and discuss treatment plans. Or a complicated medical case may require some research and consultation with specialists (that's at least a couple more projects there). All the while, during the day, my assistants would be fielding calls from clients; these messages generate more projects.
I get overwhelmed with all these projects. Pre-GTD, I just muddle through and deal with whatever is in front of me. Of course, that means some clients may not be called back until later or only patients who are critical get attention --- more of the putting out fires approach. Now, I'm better at dealing with patients before they become critical but I have to constantly spend after office hours to process these actions to make sure they get done the next day (delegated to assistants). If I fall behind with the daily processing, I get buried.
And that's only with the medicine side of things, never mind the business side of things (financial planning, remodeling, performance review, etc...).