In the recent release of the new expansion of WotLK. Now the Horde and Alliance are fight aginst in the active battleground.
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.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.
In Valgarde, players will be re-introduced to Tirion Fordring, who is now the new incarnation of the Ashbringer and wields the legendary sword. Fordring has come a long way from when you last saw him, and he now leads the Argent Dawn and the Knights of the Silver Hand against Arthas. Unfortunately, the two factions have merged into the Argent Crusade, giving you (you guessed it!) yet another faction to grind up from neutral to exalted with.
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.
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.
Again, players who are geared in tier 5-equivelent equipment will not be seeing any big gear upgrades, but the quests completed here do net a sizeable amount of experience and reputation so they are very much worth completing.
There are 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; Tirion Fordring will eventually move from the Fjord to Icecrown, for example.
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.
Both factions will visit Whisper Gulch through quests; the Horde will gain a quest called "Brains! Brains! Brains!" and the Alliance will get "Danger! Explosives!" Whisper Gulch is a site where you will find a whole slew of insane dwarves. Adding to the creepy atmosphere, you will receive several whispers from "Unknown Voices," giving you encouraging remarks such as "you are a pawn of forces unseen," or "there is no escape. Not in this life, not in the next." For those of you who have been in the Temple of Ahn'Quiraj, you've encountered this sort of thing before, as C'thun will periodically whisper similar menacing messages to you. This ties in with a plot thread you will encounter later on in the Grizzly Hills, and again in Azjol-Nerub relating to an Old God who has awakened and is starting to gather up followers.
Utgarde Keep can be seen in the distance from both Valgarde and New Agamand, a constant reminder of the threat from the Vyrkul and their Scourge masters. Players just starting out in Northrend can immediately go straight into Utgarde Keep, as it is designed for players ranged from level 70 to 72. It has three bosses and is designed to be a short run, possibly taking less than 45 minutes to complete assuming everyone does their job well and there are no wipes.
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.