For me, the hardest part about inbox-zero is that I use my inbox as a todo list. And a lot (most?) of my job involves reading an E-mail where Alice has a problem and needs me to E-mail Bob to get them to do something. So now I have two things in my todo list: 1. hound Bob until he finally reads his E-mail and does the thing, and 2. follow up with Alice to let her know the thing is done. I haven't yet found any great way besides email-as-todo list to manage this. Multiply this by 15-30 things at a time, and I end up with at least 30-60 things in my inbox.
Gmail's "stars" are an OK solution, but I tend to forget to unstar things when they're done, so I need to periodically do star-cleanups. And copy pasting tasks from my inbox into some other specialized todo tool has never really been worth the trouble. The todo apps just aren't integrated enough with the rest of my E-mail based workflow. And there's never a point where I'm not waiting on someone so I never really get to zero.
Gmail's "stars" are an OK solution, but I tend to forget to unstar things when they're done, so I need to periodically do star-cleanups. And copy pasting tasks from my inbox into some other specialized todo tool has never really been worth the trouble. The todo apps just aren't integrated enough with the rest of my E-mail based workflow. And there's never a point where I'm not waiting on someone so I never really get to zero.