The Horde really doesn't have many "big epic questlines", do they? Or if they do they're shareable with the Alliance (DK opening, Darrowshire). The only real "big, slightly epic storyline" I picked up on was the Dar'Khan Drathir questline in the Ghostlands, the second Blood Elf zone and the home of Zul'Aman. Story goes, he [Dar'Khan] turned traitor and allowed the Scourge into Quel'thalas in return for the power of the Sunwell, power goes bye-bye when Kel'Thuzad is resurrected, he's pissed, dragon turns him into cinders, Lich King resurrects him as a puny level 21 elite undead mob that I regularly kill for fun and to level up weapons skills (by killing any mob in sight in the Dead Scar, from Silvermoon to Deatholme) and you go kill him and subsequently score the BEs a place in the Horde. I say "slightly epic" because it's pretty short and it never leaves the zone until AFTER you've actually killed the guy and turned his head in for a quest blue, compared to the Darrowshire one where you're running all over the place like the Eastern/Western Plaguelands and Winterspring of all places (Talk about a remote zone! Last time I was there I was gathering chickenbear (Owlkin) blood for my warlock's dreadsteed quest).
I even consider the Moria Bronzebeard quest chain slightly more epic than the Dar'Khan line because it involves saving a princess, not an unfamiliar sight to games, but because of the outcome, in that Moria's carrying the unborn baby of her and the Dark Iron king. Again, both Horde and Alliance can do this, but it's more signifigant to the Alliance because A: Moria's old man is the king of Ironforge (Magni Bronzebeard), and B: this means that at some point there will be a Dark Iron dwarf, most likely with the last name Thaurissan in Ironforge's halls, something that hasn't been seen since before the War of the Three Hammers, if any of you non-lore-nerds are listening (DO A DAMN WOWWIKI SEARCH!). To the Horde, it just means negotiations are gonna be harder. To the Alliance, it could be catastrophic because the Dark Irons are so hellbent on destroying and recapturing Ironforge.
The Punchmaster. Do Not Question Him.