There are another couple of limitations that I think are worth mentioning.
On a physiological note there's bone proportion. Sometimes in the yoga class I hear "I can't do that, my arms are too short!" And then people chuckle. But there very may well be truth in a comment like that. Lets take the humerus for example - (the bone in your arm that goes from your elbow to your shoulder). If you are trying to do a headstand and your humerus is relatively short you may literally not have enough space to lift your head from the ground (in Rilley's video "Anatomy for yoga" you will see someone with this particular problem. And if you can't get hold of the dvd imagine a baby trying to do a headstand and you'll get the picture) . So sometimes it has nothing to do with lack of strength, will or practice it's simply as in this case, proportion.
Have you noticed that doing yoga in the morning (apart from being a wonderful way to start the day) is often "harder"? - you feel more rigid and stiff - certain asana that seem easy in the evening suddenly become more challenging after breakfast. (ooops - obviously one attemps yoga on an empty stomach only!). This is due to "ground substance" . It fills the spaces between connective tissues (bone , cartilage, tendons and ligaments etc.) and along with other elements helps slippage and lubrication between joints for example. Ground substance is usually fluid but it congeals and loses moisture with lack of movement causing everything to become tight and brittle; muscles lose their elasticity. So after a night's sleep where you have been relatively immobile for a long period of time, ground substance starts to lose it's moisture and therefore lubricating ability. Luckily as we go on moving throughout the day "morning stiffness" lessens due to the ground substance regaining its properties (for more information on this see "Anatomy of Yoga" by Coulter) - and yoga gets easier.
You may find in a twisting position for example, that keeping your eyes open you get to a certain point then stop. Try doing the same with your eyes shut and you may find you can twist around just a little further. Why is this? I presume it's because your sight helps you decide where you think you can get to in a position, it's used as a kind of (sometimes unreliable) yard stick. So with your eyes shut you just let your body be its own measuring tool - you push yourself to your real limit rather than to your presumed limit. You listen inwardly rather than look outwardly.
Listening inwardly is a great amplifier of your physical state. It's like looking through a magnifying glass to really find out what's going on inside there. When pregnant both times with my girls I under went a "villocentasi". It's a procedure used to discover if your unborn baby has down syndrome along with other disabilities, and one which requires the insertion of an incredibly long and surprisingly thick needle into your womb through your tummy, which then sucks up the "villi" (tiny finger like projections that line the womb) with the chromosomic information there contained. The doctor's assistant told me in no uncertain terms before the needle touched my skin; "whatever you do keep your eyes OPEN!" Through years of experience she had come to understand that by keeping your eyes shut you come far more into contact with the interior world which when affronting pain is best to be avoided. But when on the yoga mat it is best made advantage of.
On the other hand I recommend that whilst meditating one keeps ones eyes OPEN, but that's for another post!
"there's no way I'll ever be able to do that!". How many times have I heard that.... And not because I demonstrate particularly amazing or unachievable asana (I have lots of physical limitations; back bends - having little range of movement at my thoracic and lumbar spine all the particularly jaw-dropping asana are beyond my grasp- I'm not being modest nor implementing a mental unfounded limitation; physiologically that is simply the way it is) the reason is because people doubt themselves before putting themselves to the test. What a shame! My teacher always used to compare this scenario to a child learning to walk. A child will pull herself up, fall and try again fall and try and again and again (even when she's not perhaps yet physically capable of doing so) until finally she can get up on her own two legs. She would never think to say "well...there's no way I can do that!!!". Without self-critisism or self doubt she just goes for what she wants. She's not boastful, or arrogant, she's not overestimating her abilities, she's simply.....learning. Without mental barriers. I invite you to do the same.