I think defining ALL of your projects (big, little, personal, professional) is a great exercise. Then, what you actually DO decide to work on then is top quality. The Someday/Maybe list, though, is what I'd suggest you utilize more. (I have 145 on it right now.) Might as well either dump the stuff or park it on S/M so you don't have to have anything actually to DO about it.
That way, you won't feel bad about your own agreements.
Sometimes, it's easy to avoid defining the project because it's too big ("amorphous, out of my control"), too small ("some dumb, dorky little thing") or too ambiguous ("I'm supposed to do WHAT?"). Interestingly, you would only avoid moving on it if the action step isn't clear enough or the project is not meaningful enough, given all your other projects. A great way to get around that is to completely define the successful outcome of the accumulation of action steps required to complete that "thing." For every active project on that list, ensure there is a "next action" defined and captured into your system. Finally, if you're still not getting it done, then slide it off the active list.
Also, keep in mind that having a "complete" projects list forces (and makes easier!) the priority question. "What's most important for me to focus on?)