Beachhead Zone

Of late, as the release of the new expansion of Wrath of the Lich King launches, in the active battleground the Horde and Alliance are fight against.
On the Horde side, the first place you will see is Vengeance Landing, a site controlled by the Hand of Vengeance, a faction of the Forsaken sent to Northrend by Sylvanas for the specific purpose of overseeing the release of the Forsaken's "New Plague," which Horde players have periodically assisted with via quest lines. Some of you know where that plot thread eventually leads, but I won't spoil it for the rest of you. Just like the Argent Crusade, the Hand of Vengeance is another faction to grind up to exalted, with some decent rewards for doing so.
For the Alliance, the first major quest line you will be involved with is "Hell Has Frozen Over," which immediately pits you against the Vyrkul that are attacking the walls of Valgarde. This is the quest that will nab you the first green items (assuming one doesn't drop off the Vyrkul you kill) you will see in the expansion. Surprisingly enough, the green items you will first see are not much better than Karazhan epics. In fact, if you are completely Karazhan geared, you most likely won't be replacing your gear until you hit level 72 or 73. This is a big difference from the transition from WoW Classic to the Burning Crusade, as players with Black Wing Lair gear started replacing their epics as soon as they started doing Hellfire Peninsula with random green item drops. In Wrath, however, your epics will last you quite some time; the same goes for Arena gear for Seasons 3 and up.
An Alliance quest line that starts with "The Explorer's League Outpost" will eventually take the player to Baelgun's Excavation site, where the player will encounter Loken, the final boss of Halls of Lightning and a major lore character.
One of the first quest lines you will take up as a Horde player will begin with a quest called "War is Hell," which will pit you against a group of Alliance battle ships off the coast of New Agamand, which have been attacking the Forsaken for several days. Upon completion of this quest line, you are immediately introduced to Prince Keleseth, the first boss of Utgarde Keep and see a cut scene that spills a lot of lore.
Both factions have two quests that will lead players into Utgarde. The Alliance quests are "Disarmament" and "Into Utgarde!" which ask players to steal Vrykul axes and kill the final boss of Utgarde Keep, respectfully. The Horde quests are "A Score to Settle" and "Ingvar Must Die!" which have players kill the first and last bosses of the Keep, respectfully. Both quests will net a decent amount of experience and reputation, so even if the rewards aren’t quite upgrades for you, they’re still worth doing.
Through quests, the Horde will gain a quest called "Brains! Brains! Brains!" and the Alliance will get "Danger! Explosives!" There're several quests and encounters in Howling Fjord that serve as previews to things you will see in other zones at higher levels. Everywhere in the Fjord you will see servants of Arthas and discussion of plot points that you won't see culminate until you reach Icecrown Glacier; Eventually Tirion Fordring will move from the Fjord to Icecrown.