Depends if it's multiplayer or not. Half of the time in a multiplayer matchup I can play my tiered Legacy deck and still manage to lose. Of course I'm not being a dick at the same time, but it's in the eye of the beholder.
1 on 1, casual consists of no Swords of XXX and YYY, No Jitte, there are a select few OP as crap cards that I can name as well (Wasteland, Entomb, NO, and quite a few of the cards on the banned list)
Of course, there's still some exceptions. Like Earthcraft isn't the worlds most OP card, and can be quite fun. While on the other hand if I beat your ass down with an affinity deck complete with Cranial Plating and Skullclamps you're gonna wanna punch me in the face.
I think if your deck wins fast, it's probably not casual.
Also, I'd personally consider a commons deck to be always a casual deck (a deck that ONLY has commons, I have a few of them now). Yeh, there's a few combos in commons that can win on turn 2... But... They're so loose and flimsy that any kind of permission (counter, sacrifice, even -1/-0 for one of them would do it).
Bottom line is, Casual means you've built the deck for fun; not exactly to win.