World Events

Instead of just having world events that relate to an expansion, or patch, have weekly or biweekly world events.  They could be attacks on central cities, or attacks on small towns in places, by NPCs.  For example; Centaur could try to invade Thunderbluff; Demons could pour out of Ragefire chasm into Ogrimmar; pirates, murlocks, or naga could attack the Darkspear Islands; the Scourge, or Scarlet Crusade could attack Undercity; trolls could march on Silvermoon City; Leper gnomes and Troggs could attck New Tinkertown; thugs, gnolls, or fire elementals could pour out of the Stormwind Stockades; black dragons, satyrs, nightmare green dragons could attack Darnassus; there could be a Dark Iron clan revolt in Iron Forge; and the Burning Crusade could attempt to corrupt and take the populate of Exodar.  And these events could be done in such a way that you could only damage and aggro mobs and the bosses around your character's level.  A level 0 wouldn't be able to get raped by the ?? boss of the raid.  And it wouldn't just have to start at the city.  The raid could march on the city, going across several zones.  And there wouldn't have to be a huge announcement, it could just happen.
There could also be natural disasters, and the relief that goes with.  Fires, floods, earthquakes, storms, rockslides, etc, could happen in zones, to towns.  This could have stuff like during holiday events, like putting out fires, moving rocks, bandaging hurt people, bring/creating food, etc. 
These kinds of things could make all the zones and places more fun to go to, and that gives everyone more stuff to do instead of grind.