Korozda Guildmage – Return to Ravnica uncommon. The guildmages return in Return to Ravnica, but this time they're all new names and new mechanics. Also, you'll see that these guildmages don't have hybrid mana, and instead require one of each of the guild's mana colors. Colloquially they were sometimes called gold-mages in R&D.
Izzet Charm – Return to Ravnica uncommon. Charms are most recently remembered from the Alara block. They were so successful and well received there that they were brought into Ravnica, thus giving every guild their own charm. You can use the Izzet charm in Duel Decks: Izzet vs Golgari.
Dreg Mangler – Return to Ravnica uncommon. Dreg Mangler is a three-mana 3/3 with haste and it doesn't require any red Mana; that's notable right there. Additionally it features the Golgari mechanic of "scavenge." Each guild mechanic in Return to Ravnica was designed with the original Ravnica mechanics in mind, scavenge plays quite nicely with dredge, both of which are used in the Golgari duel deck.
Street Spasm – Return to Ravnica uncommon. As the first reveal of the new Izzet mechanic "overload," Street Spasm is in Duel Decks: Izzet vs Golgari. Overload was first introduced by Ken Nagle in the Great Designer Search, which he then called "dispersion." Ken led development on Return to Ravnica and thus was able to get "dispersion" in as the Izzet mechanic.
Jarad, Golgari Lich Lord – Return to Ravnica rare. Jarad was the first new card revealed in Return to Ravnica. He is the new Golgari guild leader and one of the two premium foil cards in the Duel Decks: Izzet vs Golgari. He will also be only the third Zombie Elf to ever be printed, joining Glissa, the Traitor and Llanowar Dead.
Desert – From the Vault: Realms rare. In Alpha and Beta, all the nonbasic lands were rare. However, Arabian Nights brought the first common nonbasic land into existence in the form of Desert. We don't have the record of the entire decision-making process of making it a common, but presumably the fact that deserts are, well, all over the setting, figured in.
Kresh the Bloodbraided – From the Vault: Legends rare. Kresh's braids are tokens of success in the hunt, and he has more than anyone. Although he is not the leader of his tribe, he is universally recognized as the greatest human warrior alive.
Ivory Tower – From the Vault: Relics rare. Life gain no good in constructed? Ivory Tower appeared in the finals of the World Championships three years in a row. Zak Dolan had one in his winning deck in 1994, and Marc Hernandez and Mark Justice finished second in '95 and '96 respectively with the Tower.
Necropotence – From the Vault: Exiled rare. Necropotence dominated tournament Magic almost from the day Ice Age came out, leading to the "Black Summer" of 1996. In 1999, combo decks like High Tide and Academy ruled the landscape to the point where Necropotence wasn't in any decks. At that point, Chris Pikula made the famous suggestion, "Ban everything until Necro is good, then ban Necro". And that's pretty much what happened.
Form of the Dragon – From the Vault: Dragons rare. The “Moat-like” effect of this card would normally be associated with white, but for flavor reasons the ability was allowed on this red card as a rare exception.
Watercourser – Magic 2013 common. The Elemental in the art for Watercourser is not flying. It's rising up out of the river in response to the rain!
Fencer Clique – Morningtide common. Morningtide was a set where what mattered was class, rather than race. So while Fencer Clique and Vendilion Clique were both Faeries, it was more important that one was a Wizard and the other was a Soldier.
Index – Magic 2013 common. The Magic 2013 version of Index has the same art and printed rules text as every other version of the card (it was printed in Apocalypse, Eighth Edition, and Ninth Edition). But the flavor text has changed, possibly because the original version referenced both Mercadia and mercenaries, neither of which are in Magic 2013.
Fiery Conclusion – Ravnica: City of Guilds common. Single-color cards in Ravnica block could belong to any guild that contains that color. This is demonstrated by the flavor text on Fiery Conclusion, which describes the same effect from the point of view of three different red-aligned guilds.
Kessig Malcontents – Avacyn Restored uncommon. Originally, Kessig Malcontents was only about a single malcontent, but the mob-like art and ability lent themselves to pluralization.
Cloud Cover – Planeshift rare. Cloud Cover has been around for a long time, and it's always seemed ripe for some kind of crazy combo. Possibly in combination with Azorius Aethermage, right? But it never really broke through.