You can gear your char up, but not use any passive skills, or active skills except for the skill given to you at level 1.
Barbarians - Bash
Demon Hunter - Hungering Arrow
Monk - Fists of Thunder
Witch Doctor - Poison Dart
Wizard - Magic Missle
You cannot play in a party with other people unless they are doing the challenge as well. You can use a mercenary if playing solo, but your mercenary cannot have any skills chosen.
Or, maybe... only 1 Primary Skill with any rune?