I weighed in on this in an earlier thread, however, for convenience I'll paraphrase here:
Death Angel Rulebook wrote:
Players may share any information they wish about the Action cards in their hand but may not show cards in their hand to other players.
As I understand the rule it indicates (like in Pandemic) that you are allowed to tell the other players as much as you want about what you have, but you cannot show them your cards, they have to communicate with you to find out (which red cards do you have again? instead of since you have Jakarta fly there...)
In Death Angel this is even more important since the players' actions can affect the outcome of other players' chosen actions (depending on the order of resolution, movement or other special abilities). The rule may sound silly (why can't we just play with open hands?) but it is trying to encourage communication between players (a large part of the fun of co-ops- for me).
That's my take anyway...