According to wow-heroes, Kilrogg has over 265 guilds on it. If we multiply that by 111 (the number of seperate realms), we come up with a number around the 30, 000 mark. That is a lot of guilds. Of course, some of those listed will now be defunct, but it's also not counting those guilds that have never really attained an achievement that would see their name pop up. It's hardly surprising that people have a rough ride picking one.
I expect most people are in a guild. If you're not, you're in the minority. But people are often in the wrong guilds for what they want with regard to the game, as there is such a wide demographic and spectrum of choice. This makes picking the right guild difficult, because you will often join one and find that it wasn't exactly what it said on the tin. This isn't deceptive or dishonest, but guilds are essentially created at the whim of their members and this is what makes them so eclectic and, oftentimes, prone to sudden change. To that end, it's maybe worth me writing down what I think would be a helpful "tick sheet" to help anyone in their decision regarding which guild to join. I would say that there are five main types of guild (and numerous shades of grey in between) and which you choose should enhance your World of Warcraft experience. If it isn't doing so, the chances are good that you joined the wrong type of guild.
Okay; here's how I see it (bearing in mind a heavy PvE bias):
The Top 1%
These guilds are the best of the best, fighting for the world firsts against everyone else playing the game. They demand high levels of commitment, large quantities of time and members are expected to show up and perform, on demand. Application into these guilds is solely based on available spots with regard to competition, and the process can take weeks. You are also likely to be trialled before becoming a full member and the vast majority of people will fail this trial. Do not apply unless you compare favourably with the very best and can commit pretty much every evening to World of Warcraft. Leadership is generally entrenched and consistent in its demands, with little deviation. You either shape up, or ship out.
The Main Event
The "Main Event" guilds are usually the best on their server and will typically be in and around the top ten of the region. Not seeing a server first will be the exception as opposed to the rule and while they may not be as accomplished as the top 1%, they certainly won't carry anyone and this is what sets them apart. Applications to these guilds will normally take around a week and, as above, you'll be trialled; sometimes silently. Generally, if you're thinking of applying to these guilds, you should have played with a few of its members and compared favourably to them, while maximising your character and being able to commit the time. Leadership is similar to the top 1%, but a little more subjective in practice.
A solid progression guild, moving at a slower pace than the server's best, is what typically characterises this group. Invariably this will see a solid raiding core at the top that changes little in the short term, and can accept a certain amount of slack on behalf of its members. The membership demographic is invariably wider than in any other group, as it will encompass those who want to push the server's best guilds, those who just want to raid with a reasonable amount of success, and those who really only want to play with their friends. Applications usually only take a couple of days at the most and leadership is usually charismatic, sometimes seeing officer positions change hands with relative frequency in an effort to keep the majority happy.
The Front Row
These are the casual guilds that aren't really involved in progression, but set up raids for certain members and will usually incorporate PuG players that are known to members of the guild. Any progress will be made in content that is significantly outgeared, if at all and raids will be organised on a whimsical, ad-hoc basis. It's common for members of these guilds to do most of their gearing in PuG's and they are organised around a high volume of social interaction. Leadership in these guilds is practically always loose, ill-defined and sporadic but also typically the most friendly and accommodating. Membership application will consist of nothing more than whispering a current member and asking for an invitation.
The Cheap Seats
Purely and simply, this type of guild is a glorified chat room. Any character development is done elsewhere, and some of the members may well be ones who jump into raids organised by the Front Row. From time to time you'll get someone who'll set up a raid in easy to access and easy to clear content, but you won't find any progression plans whatsoever. Officers will always be the chums of the GM, chosen for no other reason; but the leadership overall is generally the friendliest and most welcoming to people joining the guild and the atmosphere should be fun and free of drama. As with the Front Row, a whisper is typically all you need for an invitation.
The Parking Lot (Sub-category)
If you're in a Parking Lot guild, you'll have no long term plan, no leadership and no hope. There are literally thousands of these guilds and the players you find in them are typically alts, bankers, or long-forgotten players that never really wanted to get involved. A large percentage would have started out as "the next big thing", only for the inattentive (and usually young/immature) GM to get bored with it the second he realised that you don't recruit a full raid of awesome players in two days. The rest is made up of "funny" names that ceased being funny after five minutes, "chum guilds" for those who are friends in real life, and squads of new players who simply agreed to join when whispered because they didn't really know any better. You don't ask these guilds for an invitation, they typically ask you if you want one; the answer, ten times out of ten, should be "no".
Please understand that this is not exhaustive. I should stress that there are shades of grey in between these general levels, and many guilds will incorporate characteristics of two, sometimes even three, other guild types. Each guild, after all, is different. But in practically every guild you come across, the widest spread of their players will fit into one of the above groups, with a bit of skirt width on the right and left of arc to smooth out the edges. The point is that you should think about what type of guild you're looking for and, subsequently, what guilds on your server will cater for what you consider most important. The higher up the scale you go, however, the more emphasis is placed on the vice versa:
What is that guild looking for with YOU?