One phone call. That's all that came between Magic as it is now, and a less elegant game with thirty different card backs.
Arabian Nights almost had a completely different card back (as you can read about in Richard Garfield's article), but an eleventh-hour change of heart (partially due to player protests) and a last-minute phone call insured that Magic's first expansion had identical card backs to the base game. If that hadn't happened, the precedent would have led all expansions to have different backs, and who knows how matches would have played out with all that extra information… ("Hmmm… my next card is from Legends… must be Mana Drain.")
Here is a standard card back, and the lovely purple-and-gold design that was proposed for Arabian Nights.