Completed tasks are an important part of the Task list.
They need to be shown to the user as it provides satisfaction that tasks are being completed, making user more productive and in return use Wunderlist much more
Even though tasks might be in different categories (inbox, today etc...), Completed tasks list should just put them all together and provide a filter per user and per category
The most important part of self-organiser!
Could not agree more. This is a deal breaker for me