Alliance has tons of rich lore. Its not a question of it not being there or not being strong enough to stand on its own. Its all down to Blizzard to actually use it and craft it. There are tons of great lore characters, Alliance specific they can draw upon and use. Such examples as Turalyon, Alleria, Khadgar and Danath - could all be brought back and used to great affect. I'd also say
The biggest problem is Blizzard try to make content for both factions. Siege of Orgrimmar was probably initially very Alliance centric, until Blizzard adapted it in order for it to make sense for Horde players to lay siege to it. Then it naturally went from being an Alliance story, to a Horde one. Especially so since 5.3 was all about the Horde rebels. Had they actually bothered to craft Alliance specific quests to undermine Garrosh's forces without aiding the rebels, it probably would have worked out better storywise.
Its also very likely that if content such as Kul' Tiras is ever introduced to the game, Blizzard will attempt to butcher it so it makes sense for Horde to be there, similar in a lot of ways to Dalaran.