Reply #1: Guild choice can be tricky, as there are some good guilds out there...

[Hong Kong Cavalier]

...and some very, very bad guilds. It depends on what you're looking to get out of the game, actually. If you're planning on doing the "end-game" content (raiding) you may want to seek out a guild.

Some raiding guilds are "casual" raiding guilds (like the one I was a member of for two years) and don't have a super-urgent competitive drive to raid the latest and greatest content. The raiding guilds that call themselves "hardcore" raiding guilds tend to be more...obsessive about raiding. Membership is usually stricter there, and they expect you to attend scheduled raids (often more than four times a week) in order to "keep your spot" in the raid.

Note that this all varies by guild, actually. It's nearly impossible to completely categorize each guild. Most guilds will have corresponding websites for applicants and discussions outside of the normal gametime. Most well-organized guilds will have their "mission statement" on their website for potential guild members to review.

The best way to determine if a guild is a good fit is if you play with them. All guilds are limited to the realm where they're located; there are no cross-server guilds (at least they can't really communicate with each other across servers). Getting to know various people in various guilds on the server you're on is the best way to find a guild you may want to join.

Before Cataclysm, guilds really only offered another chat channel and a bit of event scheduling assistance, but with the introduction of guild levels in the new expansion, joining a guild can be rather beneficial. The guilds can "level" as its members gain levels and reputation, adding benefits to all guild members such as +5% xp or rep gain (or even skill gain) and even guild mounts. The more quests you do, the more your reputation with the guild increases and the more benefits you can enjoy

Any other questions, feel free to ask.

Disclaimer: I should tell you that while I do not play WoW right now, I was an officer in a guild for over a year and a half. I left the game for personal reasons (Not having to do with my guild at all. They're great people)