The inspiration of the Scourge's architecture lies in the ruined catacombs of the Nerubians, sentient arachnids long since conquered (and promptly resurrected) by the Lich King. Now controlled by the Scourge and the mysterious Forgotten Ones, this subterranean zone was left untouched during Arthas' battles with the Nerubians of old - so expect instances, and plenty of loot. You'll also be able to visit the few remaining living Nerubians in the Old Kingdom, where the Dwarves are attempting to keep the entrance of the city shut as protection from the Undead.
The west-most corner of the continent houses an icy entry point into Northrend. The Horde arrive into Warsong Hold, under the command of (deceased Horde champion from Warcraft III) Grom Hellscream's son, Garrosh, and the Alliance into the vast Justice Keep on a massive steamship, with both factions battling away against the Naga. Living in Riplash Ruins, the fishy former-Highborne (posh elves to the layman) seek to flood the Tundra with a gigantic ice-melting machine in a '50s supervillain fashion. The other notable faction are the Walrus-esque Tuskarr, who ally themselves with the Horde, eat whales and share similar tribal habits. As well as similar bodily hygiene.
Housing the Drakkari ice trolls, Zul'Drak is a huge valley full of little villages, the Troll capital of Gundrak, and the ruins of those conquered in the civil wars leading up to the creation of the Drakkari nation. These Ice Trolls are bigger, meaner and more barbaric than those you've encountered before, inhabiting various smaller tribes under the banner of Frost King Malakk. This is inhospitable territory for both Horde and Alliance, steeped in ancient, newly uncovered lore and angry, icy fauna.
The Grizzly Hills are like Ashenvale meets the Hinterlands, populated with all manner of Furbolgs, Ice Trolls, and even the ever-present threat of the Venture Co., seeking to turn a hasty profit in the war against the Scourge. The Dwarven camp of Thor Modan stands to the north-east, giving the Alliance respite from the cold and the constant bear attacks. Expect giant trees, rolling hills, and a big PvP fight, much like the Arenas in Hellfire Peninsula. Bordering the zone is the lower-level (72-74) part of the instanced Drak'Tharon Keep.
This two-part instance leads in from two zones, The Grizzly Hills (leading to the 72-74 wing) and Zul'Drak (leading to the 74-76 wing). This former Ice Troll citadel has been home to the Scourge, so there'll be some undead Troll action just waiting for you. This keep also formerly held Frostmourne, the runeblade that turned Arthas into a Death Knight, and drove him insane.
To the east is the lush Howling Fjord, proving that Northrend isn't purely ice. Reached through Daggercap Bay (where Arthas first arrived), the Alliance stop in at Valgarde by boat from an unknown (as of yet) location, and the Horde reach New Agamand - named after the Agamand Family found in the undead starter zone Tirisfal Glades, thus inspiring the new Tim Burton-style Forsaken architecture. With verdant areas at water's edge, moving up into snowy vistas and pine forests that have been set ablaze - it's a beautiful area. Both Horde and Alliance will be called to deal with a bunch of newly awakened half-giant Vrykul, attacking from the village of Nifflevar and the instanced content at Utgarde Keep.
Leading in from the Howling Fjord, the Keep (similar in many ways to Hellfire Citadel) is populated by the Vrykul, notably the Dragonflayer Clan, who are arming themselves to do battle with the Alliance and Horde. Made up of three wings, the keep has content for the higher 10 levels, such as the Catacombs (levels 70-72), Pinnacle (level 80), and the third wing is pure raid content, in all likelihood placing you in battle with King Ymiron, king of the Vrykuls. And local Scrabble champion.