Question Elemental? – Unhinged uncommon. The original version of Question Elemental? was in the never-published Unglued II, where it was called "Jeopardy" after the popular US game show. The art, which showed members of the Weatherlight crew playing the game, can be seen here.
Careful Study – Odyssey common. The mechanic of "drawing cards" has been depicted many different ways on Magic cards, going back to Ancestral Recall's somewhat abstract representation of "remembering what your ancestors knew." The most common version, seen on Careful Study, is that drawing cards represents knowledge.
Gruesome Discovery – Dark Ascension common. The scene portrayed in the art for Gruesome Discovery is a Innistrad nobleman, whose wife has just found his corpse. The artist has chosen not to depict the horrific violence directly, instead allowing the wife's reaction to convey the horror she feels.
Trenching Steed – Prophecy common. Trenching Steed is what you get when a farm horse, used for plowing trenches in fields, is put to work fighting the oppressor and jumping trenches of men. This week's Card of the Day theme is "Oscar-nominated movies" and that seems perfectly fitting for Spielberg’s War Horse.
Crystal Ball – Magic 2011 common. Once core sets started having new cards in them, R&D was able to use the opportunity to present simple and clean implementation of mechanics. And when scry was added to Magic 2011, that let Crystal Ball present a very flavorful version of it.
Goblin Mime – Unhinged common. Goblin Mime was one of five Unhinged cards that had alternate-art promo versions. You can see the other version of the art here, along with the other four cards.
Weirding Shaman – Morningtide rare. As the flavor text on Weirding Shaman implies, the goblins on Lorwyn are organized into tribes each ruled by an auntie (like Mad Auntie or Wort, Boggart Auntie). Since the aunties are all shamans, this means that the male goblins are all magicians' nephews. The female ones are magicians' nieces.
Sigil of the Empty Throne – Conflux rare. At the time that Conflux is set, when the five shards of alara begin to merge, the angel Asha has been dead for centuries. But no one replaced her as leader of all the angels of Bant, so her throne remained empty.
Dawntreader Elk – Dark Ascension common. There are four elk in Magic. All of them were printed with the creature type "elk" except for Gang of Elk, which obviously needed to be included.
Prismatic Wardrobe – Unglued common. The text box for Prismatic Wardrobe contains sections from the backgrounds normally assigned to each individual color. It also has land and artifact sections in there, even though those aren't strictly relevant to what the card does.
Mtenda Lion – Mirage common. Through almost the whole playtesting process, Mtenda Lion was unable to block blue creatures, to echo Gibbering Hyenas's inability to block black creatures. It was changed at the last minute because the blue decks of the era didn't play creatures.
Increasing Vengeance – Dark Ascension rare. From the Dark Ascension FAQ: If the spell Increasing Vengeance copies is modal (that is, it says "Choose one --" or the like), the copies will have the same mode(s). You can't choose different ones. If the spell Increasing Vengeance copies has an X whose value was determined as it was cast (like Earthquake does), the copy has the same value of X.
Sorin, Lord of Innistrad – Dark Ascension mythic rare. From the Dark Ascension FAQ: If a creature with undying is destroyed by Sorin, Lord of Innistrad's third ability, the undying ability will trigger, but the creature will already have been returned to the battlefield by Sorin. The undying ability won't do anything when it resolves.
Wolfhunter's Quiver – Dark Ascension uncommon. From the Dark Ascension FAQ: For each of the activated abilities, the equipped creature is the source of the ability and the damage. For example, if the equipped creature is not an artifact, you could activate either ability targeting a creature with protection from artifacts.