I like Todoist's simplicity. I organize my to do list in multiple categories, and each item has a due date. I don't bother with task priorities. I use indenting to group sub-items together.
Todoist's biggest problem is its mobile presentation. This is a Big Deal to me. Moving from Pocket PC to iPhone, I needed a mobile to do list. Todoist works on iPhone's web browser, but it is subpar. The task names are too small to read unless you zoom in, but when you zoom in, you have to do too much horizontal scrolling.
Remember the Milk announced a new native iPhone app, so I tried it again. This time I got it, and I like it, even without the native iPhone app. Oh, that makes sense now: each task has an info square on the right, and that's where you set the due date. You can annotate your tasks with notes, URLs, and other task information, an improvement over Todoist. It has Google Gears support so my to do list is available when my PC is disconnected. The mobile web adaptation is crude, but way better than Todoist: it is completely usable on both the web and on the mobile web.
I moved my to do list from Todoist to Remember the Milk, and so far I'm happy with it.