More than nostalgia of Old world content, more than my love of new challenging and entertaining raid content (and I do love it, perhaps too much), and more than the addition of more lore that my friends and I can debate and interpret constantly anew, I am most excited about the changes and modifications to guilds, and the ability to do 10 man raids and still remain competitive with 25man guilds.
With perks, guild recipes, and even Guild Reputation, there will be more reasons than ever to join a good guild (rather than make one that serves no purpose other than spreading the raider population around ineffectively, and the revamp to 10 vs 25 man loot policies means that there will be even less reason to leave, ensuring the the odd couple friends, one progressive and one casual, dont have to ruin their friendship over progress, nor break the guild up with drama...oh how I wish these changes had been implemented sooner...