As you rightly pointed out, Yoga attracts people who want a less rigorous way of being healthy.
Ironically Yoga was created as one of the most disciplined arts and was hand in glove with nerve-wrecking penance. That's what most people who want to do yoga miss nowadays, they don't really know what it was for , they just get into it because it's easy, and that ends up being a big mistake because it was made to not be easy.
Also funnily enough, even if people practice yoga on their own, it was developed with the inherent fail-safe of body pain in mind, so it wouldn't harm them. However nowadays you have expert teachers who push you beyond your body's limits and that's truly dangerous.
So to sum up, if you want to get into Yoga, get a good book and do it on your own.
OR
Make sure you find a really really good teacher and do a thorough check before joining a class, and don't just enter into a Yoga program on a whim.
Those are the only two ways I see to stay safe, and still benefit from learning Yoga.
Wow.. NO NO NO, DO NOT do it on your own at first. The second tip however, find a good teacher is a MUST..
Doing yoga by yourself with no experience and just a book and no one to help you understand proper alignment in your foundation poses at first is a great way to hurt yourself at worst, or get nothing out of the effort at best.
Which is why I said get a good book. As I said earlier Yoga was designed with a fail-safe (although I cannot cite references right now). Unless you just trudge through the discomfort in your body, you will not get hurt doing it on your own.
Sorry but this is false. Sensing pain can mean it's already too late and damage is already occurring.
Would you learn to fly a plane from a book? "When you are upside down in a flat spin, you may be in trouble"..
Get a good teacher who can guide you through the foundation alignments and do hands-on adjustments when you are starting out. A book can certainly help but it can't be your only guide.
Pain isn't necessarily a bad thing and doesn't necessarily mean 'damage is already occurring'.
If you are remotely in touch with your own body and athletic you can teach yourself yoga from books/youtube/podcasts. The best progress I had doing yoga was when I was living in Taiwan and 90% of the class was in Mandarin (which I don't speak). It caused me to focus more and listen to my body.
The problem is that you can't tell that you're doing a posture wrong by looking at yourself (even in a mirror) or by how it feels. Beginners really need the guidance of someone with experience who can look at their postures and help them get them right.
Okay, so let me be a bit clearer.
This is the advice I got from a hermit who has been practicing Yoga for 22 years, and teaching for 14.
He specifically told me that initially Yoga (particularly Hatha Yoga) is meant to help you with your everyday postures, such as standing and sitting, and breathing correctly.
That part is easy, but it requires practice. The Yoga postures are not really the essence of Yoga for a beginner. It's only the correction of your everyday postures. That is what I said should be done alone, from a book. That is what the body can handle, and you will not be hurting yourself. Again, this is truly expert advice, and you may follow it if you feel it is.
The hard part of Yoga is undoubtedly dangerous to pursue alone, but even more dangerous to pursue with a teacher who does not know what he/she is doing. So, even though this seems like a Kobiyashi Maru, I would advise finding a really good teacher first, and if that doesn't work, and you really have to do Yoga (for whatever reasons) you're better of doing it on your own, with instructions from a really good book. By no means do I say doing it alone should be your first option.
Your basing your thoughts on the perception that 'yoga is gentle and easy.' This is farther from the truth: There is a spectrum. Most yoga people actually start out with are the more vigorous forms.
Starting on your own is a generally a bad idea, unless you have a friend who is experienced and can help you properly understand positioning. There's some very proper positioning you need to know to avoid injury, or creating weakness / going farther than you should initially, and everyone's body is different. There is no one book that will not have excess information than what a beginner needs to know.
DO NOT learn yoga from a book. Your body will follow the path of least resistance and trick you into doing asanas incorrectly. That's a major way people get injured or at the very least don't get full benefits from their practice. It is essential you find a teacher that knows what (s)he's doing.
Ironically Yoga was created as one of the most disciplined arts and was hand in glove with nerve-wrecking penance. That's what most people who want to do yoga miss nowadays, they don't really know what it was for , they just get into it because it's easy, and that ends up being a big mistake because it was made to not be easy.
Also funnily enough, even if people practice yoga on their own, it was developed with the inherent fail-safe of body pain in mind, so it wouldn't harm them. However nowadays you have expert teachers who push you beyond your body's limits and that's truly dangerous.
So to sum up, if you want to get into Yoga, get a good book and do it on your own. OR Make sure you find a really really good teacher and do a thorough check before joining a class, and don't just enter into a Yoga program on a whim.
Those are the only two ways I see to stay safe, and still benefit from learning Yoga.