Casual is pet decks with inner synergy or a basic theme (Tribal, Burn, Land Destruction, Mill, etc)
Funny thing is, most of my decks (serious or not) have the Mill Capacity.
One of my earliest casual decks that "went there" was an Orcish Spy- Mill Stone deck (with cheap creature enchantments to keep spy alive).
It was quite the denial deck, much to the shagrin of my Hymn To Tourach/Hippy opponent.

EDIT: I just played a casual game vs someone who used cheap 3 mana cascade spells to get their only less than 3 spells into play: HyperGenesis with LOTS OF FATTIES! Is this being discussed much? He used SSG to accelllerate (although full vintage/legacy could also use ESG).
I was torn apart by the fattiest of fatties.