Probably, it is good to look at certain examples:

http://www.casualgamesassociation.org/for_professionals_recognition.php

These are the winners of a casual game competition of the established casual publishers (and producers? I don't know.)

My guess from what I saw on some of the casual game portals like BigFishGames, your game has to be extremely polished, zazang mentioned some points, another is, the game play and help system must be extremely intuitive.

I am very cusious for instance how this one will make its way on the portals:

it one the fourth price for being the "most innovative" entry of the contest: "Dangerous High School Girls in Trouble!" and its made by Mousechief Co. USA.