list with all mails, tasks and appointments of a category

I agree. I think a view of calendar, tasks, email contacts, notes all displayed in an Outlook Today like view BUT filtered by Category would be an amazingly good feature in Outlook. The answer here of searching by category and displaying by Type is simply not practical for everyday use.

Simply do a search, select the category in question, then under Scope, select All Outlook items. You will get a list of items grouped per folder, including separate groups for Calendar items, Tasks, etc.