So this will be good for guild groups. It solves every criticism from guild only players.
It will be terrible, however, for true PUGs and PUGs in guild groups. There is a history in this game, and every MMO, where group loot is employed with PUGs, and it turns into degenerate behaviour. For example: guild leaders will tell guildies to roll need on everything you can, and then we'll just trade it to guildies only.
Personal Loot wasn't perfect, and it really just needed to be iterated upon more. Group Loot? It's as if no one in Blizzard actually PUGs.