The worst part of guild membership is, by far, watching a guild die. I've seen guilds die from poor recruiting practices, internal drama, and, in one memorable case, a disgruntled member with /gkick privileges. But the most painful death I ever saw was that of a large casual raiding guild in which I was an officer. We weren't very good, and we never progressed far, mostly because anyone decent in our guild was immediately recruited away by one of the high-end guilds on our server. We kept it together for several months, but in the end, ten of our best players (including our two tanks and the best healer) were poached away by another guild. Eventually, the guild leader and I left too.
Judging by his post on the EU General forums, Aires, GM of Flames of the Phoenix on EU-Terenas, is having the same problem. His small Kara guild is being eaten away by larger guilds who whisper his members, invite them to come to 25-mans, and then ask them if they'd like to join. Admirably, Aires does not cry about it or name names, but asks a general question: Is it ethically right to poach members from smaller guilds who don't approach you first?
Opinions seem to be divided. On one hand, few people will argue that it's "nice" to cannibalize a smaller guild, and many SSC/TK guilds who do this to "loser guilds" would scream bloody murder if a BT/Hyjal guild did the same to them. But every server also has a limited supply of players who don't suck, and new recruits do have to come from somewhere. Plus, there's no real way to steal a player who doesn't want to be stolen in the first place.
The thread also contains a rare personal opinion from a Blizzard employee. CM Vaneras says that your answer to the question basically depends on why you play the game. If you play for what Vaneras terms "shiny epics", you probably won't see anything wrong with poaching, while those who play for "the cameraderie and accomplishments of a guild" will hate it.
What do you think about guild poaching?