The rogue and warlord weren't included in the group of characters because there was only a need for 6 characters. We didn't want 2 leaders in the party at first so we just decided to go with cleric, since more players are familiar with general nature of the class.
Warlock was chosen over rogue since some information on the rogue had become available on the Wizards site, and we wanted to give the warlock some love. Plus, I really like warlocks.
The only reason why there wasn't a dragonborn PC was that we didn't have a real good representation in minis form of a dragonborn that wasn't a rare mini. I didn't want DMs to dig out a rare mini to represent a character at the convention, since minis sometimes easily walk.