With all the guild changes, it looks like Blizzard really wants us to be in guilds and to stay in them. Contrary to what seems to be floating around, this has little to do with solo PvE. Keep soloing, Blizzard will not stop you, it's what has made WoW so successful.
However you can't do it anti-socially anymore; excluding everyone else. Blizzard wants you to be in a guild and they will make you want to be in a guild. Quest alone, play in a guild, that's the new path. Or maybe this is just me trying to validate my own playstyle, hoping it is "working as intended." I mostly solo these days, but on my alts without a guild channel, I feel lonely, trapped in an empty world like some sort of post-apocalyptic nightmare movie.
On the other hand, Blizzard is indicating with cross-server PUGs that they care more about you getting into groups than grouping with your server community. No longer does your guild tag represent anything and your name will be forgotten even sooner than usual. Your ninjaing antics and trash talk won't stick with you anymore, so be as big a jerk you want.
Is that cynical? Probably. But if you were around way back then, you might remember BGs when they were server-based. You might still remember some rivals, I know at the time I knew who was on the other side of my server. The factional rivalry was a huge part of the incentive to PvP.
Larisa has a nicer perspective, Ixo has worse.
No Flying in Warlords Entire
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