The type and style of guilds have changed as the game has changed. I remember when the difference between a hard-core guild and a casual guild were miles and miles apart. These days you can have guilds that are a combination of both and everything in between. Definitions of what is "hard-core" vs what is "casual" have changed as well. I've seen descriptions of guilds ranging from raiding casual to social non-stress. What does that mean these days? Depends on you and what you think they mean. Everyone has a different idea of what all these different definitions are. I'm sure that if I polled my guild I would get several different answers depending on the person, how long they've been playing WoW, what level they are, and how long they've been in the guild.
So how do you know what guild is right for you? First and foremost determine what type of player you are and what you want to get out of the game. This website, The WoW Headhunter, is really cool. Even if you don't use it to actually find a guild it can give you some insight into the type of player you are. It covers a wide range from introvert to extrovert, goals, and game play style. Do you like to stop and chat with people? Do you like to help others? Do you want to plow through content and get down to the brass tacks of killing monsters? Do you want to kill other players? These are serious and very realistic things to take into consideration when looking for a guild. I think the mistake most new players make is getting into the game and taking the first guild invite they get without knowing what they're really getting in to.
Long term goals. One thing I think is important to determine is what your long term goals may be. I know for many people this seems far off in the distance, but it's something to think about. Are you looking for something that will help you now such as a leveling guild? or are you looking for a guild that you want to invest time in. Are you interested in joining a new guild or forming a new guild so that you can help shape and determine that guilds growth? or are you looking for a place where someone else determines the path and you are happy to follow along? If you raid or want to raid it's important to determine a few additional things. Your raid times. When are you available and how much time can you commit? How "hard-core" are you? What this means to me is, how much effort do you want to put in. This may sound crazy to some but it's very important. It is best for both sides if you determine your level of commitment and playstyle before you start applying. If you are looking to be competetive and join top raiding guilds on your server you must be willing to put in the time both in and out of game, making sure you are up to par on your class and class stats. That includes gems, enchants, and weapon and armor choices. This may also include theorycrafting and min-maxing. If that is something you're interested in and can put in the time, there are guilds out there looking for you.
Change. The best laid plans will never prepare you for life changes and how that affects your game play. For some people this isn't even a consideration. It's a game and it is enjoyed while it's there. I know when I started playing I never thought that I'd be doing as much as I am in my life or in the game as I am now. Needless to say though, life creeps in and things change. In a nutshell, at some point in time the game for you is going to change. Somewhere down the line us addicts will have to re-address our level of commitment and time to WoW. This will have an impact on your guild or guild choices down the line, and it's inevitable. You may find you've outgrown your guild or the guild has outgrown you and it's time to think about a move. That can only be determined by you with a little toon-soul searching.
I hope I've given you some food for thought, whether you are a long time player or new to the game there's a guild out there for you that you'll fall in love with.