I've been thinking about this for a while, but Rory's recent post made me want to put it into better words.
I've had a lot of conversations with people over the years who wanted to know what style of martial arts they should study. The longer I've been doing this, the more I've realized what an extraordinarily complex question that really is. There's a lot of things to consider, and lot of mistakes people make in the process.
The biggest, I would argue, is seeking styles, not coaches.
Here's the problem with styles.
Styles aren't things. Not really. They're just labels we put on our particular practices. If your stuff comes from Korea, you call it Tae Kwon Do. If it comes from Japan, you call it Shotokan. And while there are certainly schools out there who desperately enforce rigid compliance with some curriculum handed down from on high, there are many schools that do no such thing. We live in an age where information on combat sports, martial arts, and self-defense is so prevalent and so easily accessed that a martial artist must really work hard to remain completely ignorant of the way others train and teach. Many good and responsible teachers take advantage of the wide array of information out there to really expand their programs and their teaching. Which is great, but it means that the labels start to mean less.
Tae Kwon Do is, in many circles, viewed as a poor choice for self-defense training. But if you live in Ireland, I'd send you to Daragh Bolton's Tae Kwon Do school in a heartbeat...because Daragh is also a certified PDR coach, and knows his craft.
Styles are inconsistent. I've met some Krav Maga guys that seemed like nice, competent trainers. I've met others who were narcissistic ass-clowns who are dangers to themselves, their students, and society at large.
Rory writes (see the link above) about his training with a man who called what he was doing a classical martial art, but included Judo style randori (freestyle grappling) and what sounds like proto-MMA sparring in the curriculum. Many people would tell you that classical martial arts don't do those things, so if you want that kind of training, don't go to that dojo.
The style is just a label. Without styles, no one knows what to call what they do. Jeff Burger told me about an instructor of his who frequently shrugged and said "I do stuff." Accurate, maybe, but it doesn't really bring in students. You won't find "stuff" listed in the phone book.
What matters is not the style, but the coach. Muay Thai is a great system to learn for full-contact competitive fighting--but the guy who is offering you Muay Thai might be a JKD seminar nerd who took a weekend certification course, and has never produced a fighter in his life. Meanwhile, the karate school down the street that you ignored because karate guys can't fight might be a Kyokushinkai school with multiple title holders in several weight classes on local professional and amateur circuits.
The Aikido school in town might be run by an ex-cop who knows more about criminal behavior and self-defense than the Krav Maga guy who tells you he can show you how to kill a man with your thumb.
If you're looking at martial arts training, look for a coach who can take you where you want to go. Want to compete? Find a coach who produces competitors. Want to learn self-defense? Find a coach who understands the complexities of that topic, and does her level best to address them as comprehensively as possible. Want to lose weight? The Danny Devito look-alike with his extra long black belt probably can't help you: I don't care what the window decals say.
Tony Blauer likes to say "Don't mistake the trademark for the truth." That quote pretty much sums it up. Don't worry about the label; know what you want, and find a coach who can take you there.