So, what is casual? Portnow believes it's hard to define, but the quick and dirty answer is: a game that can be played in short sessions, lacks finality and is "replayable ad nauseam." Now, that could mean Bejeweled, but it may also include "hardcore" darlings like Geometry Wars.
His analysis concludes, with full recognition of how obvious the answer is: a game with simple mechanics that takes skill and time to master is the key to making casual games for the hardcore. Portnow's piece is worth reading all the way through and certainly requires some introspection as the term "casual game" becomes ever more muddled.