Abominable Treefolk Adarkar Valkyrie Adarkar Windform Alpine Guide Alpine Meadow Arctic Flats Arctic Treeline Arcum's Astrolabe Ascendant Spirit Avalanche Caller Berg Strider Blessing of Frost Blizzard Brawl Blizzard Specter Blizzard Strix Blood on the Snow Boreal Centaur Boreal Druid Boreal Griffin Boreal Outrider Boreal Shelf Centaur Omenreader Chillerpillar Chilling Shade Coldsteel Heart Conifer Wurm Cover of Winter Dark Depths Diamond Faerie Draugr Necromancer Faceless Haven Frost Augur Frost Bite Frost Marsh Frost Raptor Frostpeak Yeti Frostwalk Bastion Frostwalla Frostweb Spider Gelid Shackles Glacial Floodplain Glacial Plating Glittering Frost Goblin Rimerunner Graven Lore Grim Draugr Gutless Ghoul Hailstorm Valkyrie Highland Forest Highland Weald Ice Tunnel Ice-Fang Coatl Iceberg Cancrix Icebind Pillar Icebreaker Kraken Icehide Golem Icehide Troll Isu the Abominable Jorn, God of Winter Kaldring, the Rimestaff Karplusan Wolverine Marit Lage's Slumber Moritte of the Frost Mouth of Ronom Narfi, Betrayer King Ohran Frostfang Ohran Viper Ohran Yeti On Thin Ice Phyrexian Ironfoot Phyrexian Snowcrusher

Instant-speed interaction lets you hold up mana and react to the biggest threat at the table
Follow the instructions on the card and then discard it
Greens strength lies in big bruisers, so we need our common creatures to help us get there quicker
Combined with the frankly outrageous state of things back then (4 Ancestral Recall , anyone?) leading to prolonged game states and impasses, WotC decided that enough was enough and restricted Ivory Tower
Some need to be exerted , or have an ink cost to activate