What content types have you had trouble adding tags to? So far, I’ve managed to add a tag to everything, at least in the mobile apps and the bookmarklet. Ihaven’t had a moment to test the Chrome extension or web app.
As for sorting by due date, the only way I can think to do that effectively is some sort of combination of a title/tag modifiers and saved search/filters. Even then, things are going to get a bit convoluted. Thankfully, Springpad syncs with Google Calendar, so you can get a good visual that way.
Thanks for the comment! I’m glad it was helpful, and I’d love to know how your implementation works out for you. :D