A "wall of water" a few inches in height will wash cars away. 20 feet of water, as a wall, is simply packed with pure kinetic energy. I can't even imagine a 20 foot high wall of water and I've seen plenty of flash floods that are 2, 3 or even 5 feet in height. They are devastating themselves.
Uh maybe because Asians are 'little' people.
Ok seriously... the force contained in a wall of water 20 Feet High is .. technically speaking .. 'humongous'.
It's that physics thing, Newton's 2nd law; F=ma (force = mass x acceleration)
Remember one gallon of water weighs is 8.345lbs and a cubic foot of water weighs 62.428lbs. And the speed of this tidal wave has been estimated at 500MPH. Do the math and it's self explanatory.
Take into account that all of the places that were hit don't rise more than 5 feet above mean sea-level. So, imagine a wall of water 20 feet high that keeps on coming and coming.
You're forgetting that there are very large numbers of people living along the coasts of India, Sri Lanka, Bangladesh, Myanmar (Burma), Thailand, Malaysia and Indonesia. Even a 20-30 foot tsunami will cause huge numbers of casualties, to say the least. 
And we ended up being very lucky this time. If the earthquake had just occurred two to three hours later the death toll might have reached 100,000 or more as the beaches where the tsunamis hit would have been filled with a lot more people.
" I can't understand how a 20 foot wave can cause this much loss of life and property. Is the wave actually much bigger"
It's not just the amplitude (how high) but the duration (how wide or how thick) the wave is that determines it's destructive power. Wind driven waves can be 20 ft. high but have a relatively narrow duration of only 20 to 30 ft. These waves obviously wouldn't have the tremendous mass of a wave 20 ft. high but a mile or two in duration like a tsunami might have.
Imagine, not a small wave that you can see has a face and a back, but a wave so massive that the entire ocean suddenly rises twenty feet, submerging everything on land less than twenty feet above sea level. That is a tsunami.