I've been using Doit.im for a little while now (turned Pro) and its one of the best for what I need it to do. Its not perfect - I havent found one yet that is, although I've used Remember the Milk (too stagnant!) and toodledo (bad UI) in the past. The following would make Doit.im better for me:

1. Be able to reorder tasks on future days
I know I can reorder tasks on the "do it now" group for today, but I'd like to look at tomorrow (say) and reorder tasks there ahead of time. Something I do the night before to help me organize what needs to be done the next day.

2. Color coding
Allow tasks to have a user selectable backgroup color.

3. Better/Natural task input
One thing I miss about Remember the Milk is how natural it was to enter a task and use various tags to have them entered correctly without having to go back and modify them via a gui. Like "Buy groceries next Wednesday".

4. Performance Enhancements
This is more of a general wish, but I've noticed noticeable lag sometimes after adding/editing a task and having the screen refresh. Havent been able to pinpoint it to a time of day yet but it isnt all the time.

Thanks for listening!