What should you look for when choosing a guild? That depends on your motives. I have been in a number of guilds for different reasons, and I foresee that as I progress I may find reason to change guilds again.
First, you may choose not to join a guild. Many people find solo adventuring very satisfactory and feel no desire to join a club - for that´s all a guild is. There´s nothing you can´t accomplish as a solo adventurer. That said, there are many ways in which a guild can make things easier for you, which I´ll talk about below.
The first guild I joined, I joined for money. As simple as that. I had 5s54c in my pocket when I met a level 60 warrior who was yelling out that he would give 1g to anybody who would sign his guild charter. Let me tell you that was the easiest 1g I ever earned, and the most useful. By all means I advise you to take this offer yourself, if you're not yet in a guild. You need the money, he needs the signature. Both parties profit.
What's that all about, by the way? Why would a level 60 warrior be down in the market square in Stormwind offering gold to all and sundry to sign his charter. Surely he's more selective about who he wants in his guild? Surely the last person he wants in his guild is somebody who's just joining for the money? What's the catch? There is no catch. Or rather, the catch is, he doesn't want you in his guild - but the offer of 1g is genuine. The player who is offering money for your signature is not planning to steal anything from you (what have you got that might interest a level 60 warrior, after all). He's wanting to set up a 'banking' guild. This is a guild that will eventually only have 1 member, and that member will be his banker. Once the guild is up and running, it's probable that you and all the other signatories will be kicked out of the guild, 1g richer than when you joined it. What the guild master really wants is to be able to create a guild bank where he can store more items than he could store in his private bank account. Don't worry. Be happy. Take his 1g payment, leave when he finally has his guild up and running, and take the next guy's 1g. All parties will be happy.
Eventually you may want to join a proper guild. Maybe you just want to see what being in a guild is like. I encourage you to try a few before you settle down. Sow your wild oats. Different guilds have different flavours. Try a few out, and then decide what you like.
You'll see various advertisements in the trade and general channels from guilds who are looking for new members. These advertisements often contain a number of code words that will help you identify the kind of guild that's being advertised:
Guilds that advertise "we have our own tabard and bank tab" are generally new guilds - it's a big deal for them that they have their own tabard and bank tab because the guild master and officers have just spent a bucketload of money getting them. New guilds often die out. Try not to join a new guild until you have had experience of an established guild. I don't mean that you should never join a new guild. In fact, I think that eventually you should. Just not until you know what an experienced guild has to offer.
Guilds that use the words "progression" or "raiding" should be of no interest to you until you are level 80. You will certainly be of no interest to them.
Guilds that are "friendly" or "social" are certainly worth experiencing. Sometimes this is code for "not interested in joining the rat-race to level 80 and beyond. Just interested in having a good time, sharing some jokes, hanging out and having fun". There's nothing wrong with that, and it's worth trying out.
Guilds that are "levelling" guilds can be both friendly and social, but what their members have in common is this: they are very interested in joining the rat-race, and reaching level 80 and beyond. There's nothing wrong with that, and it's worth trying out. Very often, these are new guilds who have got beyond the "have tabard and bank tab" stage, but have no members at level 80 yet.
In my opinion, you should try out several guilds before choosing a long-term partner. You'll quickly realise what style suits you - social or levelling (of course most guilds are on a continuum between these two extremes). Once you are level 80, if you are considering raiding you'll perhaps look at the serious raiding/progression guilds. By then you won't need any advice from me.
Before joining a guild, I recommend you look at their member profile on the armoury. The EU armoury is at http://eu.wowarmory.com/ and the US armory is at http://www.wowarmory.com/
A levelling guild with less than 50 members will either die, or will require a lot of hard work from its members to get it up to a critical mass. On the other hand, a guild with more than 200 members might seem a bit impersonal. One of the most important things to know about guilds is that they're usually set up by a group of people who know each other well, perhaps having been together in other guilds, or perhaps grouping together often as they levelled up. Sometimes such a group can make you feel a little like an outsider. I left such a guild some time back, after a guild photo-shoot. The location chosen was the surface of the lake in Darnassus. If you couldn't walk on water? Too bad, noob. You weren't in the photo. I left the very next day.
I mentioned earlier that there are many ways in which a guild can make things easier for you. I´ve more or less covered these already: they can provide you with pals (for the crack), they can provide a ready-made pool of players around the same level as you with whom you can group and level and PvP, and after you´ve reached max level, it provides team-mates with which you can raid.
I also mentioned earlier that I think you should eventually join a new guild. Actually, the more I think about it, what I really mean is that I joined a new guild and liked it enough to want to stay. Why? Because in a new guild, you make a difference. You level up with the other new members of the guild, you get to know them, they get to know you. You make friends. Of course, this can happen in any guild, new or old. It can happen outside of a guild. All I can tell you is this. I like being in a new guild whose members are known to me. I wish it was a bigger guild, since we could more easily plan guild dungeons. I know that whatever happens to the guild (it's at the grow-or-die stage right now), I've enjoyed it.
If you ever want to start your own guild, I have one piece of advice: don't.
Just don't. It's a lot of work, and the rewards are uncertain (unless you're just creating a bank guild). Maybe it'll turn out like the guild you hoped for, maybe it won't. More than likely, there is already a guild out there that is a good match to what you want. It's probably a better match than the guild you build will eventually be! But if you insist in ignoring my advice, at least take Matticus' advice: http://www.worldofmatticus.com/guides/build-your-own-guild/
By the way, whatever guild you join, don't accept a boost. You'll regret it. More on boosting later.
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