How can you run only one task out of a playbook, for debug purposes

Use tags!

for example:

---
hosts: all
gather_facts: no

    - name: set minus o vi
  lineinfile: dest=/root/.profile state=file create=yes state=present insertafter=EOF regexp='' line='set -o vi'

  - name: create /export/data directory
    action: command mkdir -p /export/build creates=/export/data
   tags:
     - debug

- name: install all packages
  apt: pkg=$item install_recommends=yes
  with_lines: cat /var/tmp/packages.all

Running the following will only run the create directory task:

ansible-playbook setup.playbook.yaml --tags debug

Or even better, with -vvv for debug mode:

ansible-playbook -vvv setup.playbook.yaml --tags debug