I would consider the Djinn a "casual" card, basically because he's a "vanilla creature" (although a BIG one at that). He doesn't have any special abilities...
Serra and Sengir too, despite their abilities, just because back when I started, they were the defining creatures for white and black. Yeah, Serras were used in The Deck, but in my mind, they weren't "abused" in the deck, but because the Vigilance ability let them be blockers and attackers.
Heh, I'd almost consider Morphling a casual creature
because he has tons of abilities. And whe I played him the ONE time in my group, people took one look at him and their eyes glazed over. Then when I tried to do fancy combat tricks with him, their heads exploded with the math and they just scooped
And "banned" me from ever using him again (as in not formally, but subtly like ganging up on me when they saw the particular deck he was in).
Anyway, that's why we have the committee - so three people can hash out between them what's casual at the time.