I can't believe Niagara Falls- the Canadian Horseshoe- was not mentioned yet. It is both breathtaking and unforgettable. A place that's like a magnet for thoughts and soul.
My biggest WOW so far...

Just to add something not yet mentioned here - My waking view from our hotel balcony of the mist-enshrouded monasteries of Meteora,Greece, clinging to the upper reaches of towering rock monolithes - Wow ... how could something like that ever be built in such an inaccessible place! Now you can drive up, but when it was built, the only way up was a rope ladder.

waking up in the serengeti to see a pride of lions watching the tents.

India does knock u sideways - u can see the Taj Mahal in so many photos but when u see it in reality - its amazing.

One of my greatest memories is getting the train from Delhi up to the Punjab -and watching India role by....

Peru - getting to the Sun Gate and gazing down on Machu Picchu....

But think it has to be Salar de Uyuni, Bolivia -12000 sq kms of salt at 4000m up in the Andes..WOW....

The terracotta soldiers in Xi'an, China were amazing. What an amazing discovery and think about the amount of work put into building each soldier. And no soldier is the same.

Having recently traveled in the Middle East, I'll second the mention, above, of the ancient city of Petra, in Jordan.
It's all a wow, especially the Treasury building, about 140 feet tall, not built, but hand-carved out of the rock.


for a picture:

http://nabataea.net/treasury.html

My biggest "Wow" was on the Kalalau Trail, Kauai, Hawaii, looking up AND down at the Na Pali Cliffs! Incredibly beautiful, and amazing to believe that people lived their entire lives on these cliffs.

There are two that stick out in my mind:

1. St. Peter's in Rome. I have never seen anything like that in my life. I couldn't get over how tall and ornate everything was. It blew me away.

2. And a little closer to home, eating in the Eagle's Eye restaurant (highest restaurant in Canada) on top of a mountain near Golden BC last week (you take a Gondola to the top), staring out over the tops of the Rockies. We were so high up that you looked across the mountain tops, not up at them. It was amazing....

Hearing all these other "wows" makes me yearn to go visit each and every one...we're going to Costa Rica for our honeymoon this December and I'm expecting a major "wow" there too!

Machu Picchu meets all of your criteria for a WOW place. From the moment I saw a photo, I knew it was a place that I needed to go. Our experience there was made even more unforgettable by staying overnight at the far-less-than-WOW Hotel Machu Picchu. The hotel was definitely nothing special, but staying there provided the opportunity to re-enter the Machu Picchu site after hours on a full moon night. A group of "sacred site" pilgrims and their "shaman" could be heard off in the distance chanting. It was simply magical.

We just got back - the Grand Canyon.

MY

In China, the Li River in Guilin. The boats and mountains, Spectacular.

I'm with P_M on this one....Red Square at about 10:30 pm. That was one week ago tonight. It was hard to do anything but stand there and say, "wow!"

U.S.N.R.you speak for over a million soldiers,though many we have lost, TRUELLY you were part of the greatest generation in this country.My entire family 6 uncles and one aunt served in the big one .Most our gone and that makes me very sad, but I am glad I can thank you from the bottom of my heart for all you did and I am sure all you sacrificed in your life makes us damn lucky , bless you.

After reading dorgal's post. My choice is meaningless.

Oradour-sur-Glane, the french village whose entire population was murdered by the Nazi's near the end of the war. Without a population, it was turned into a memorial, and appears as it did on the day of the massacre. A little known site, that drew several wows from me.

topping
this is more recent - it may be what you are looking for

A flower shop in Kabul, Afghanistan.

Simple as that. A flower shop in a city that suffered major destruction through decades of wars.

A beautiful, <b>(wow)</b> sighting for me.

A simple sign of hope for the future.

For natural wonders, the Okavango Delta gets my vote for a WOW and for a man-made wonder, the Taj Mahal-no picture captures it's beauty.

Well, nowadays, one has to say Baghdad or Babylon. Until a couple of years back not bad places, but wow!... just look at what's happened to them since.

It's not a single place, because every trip we take we find at least half a dozen Wows. I think it is that the world has become so accessible to so many so quickly. At the turn of the century, my great-grandparents (and great-great-grandparents) were only just coming to America from Sweden and Germany. It took them months to get here, and then more travel to get to Nebraska, Illinois and Wisconsin (respectively). Now we can be there overnight.

It's amazing that I am able travel from the US and meet up with people I already know in England, Scotland, France, Belgium, Germany, Macedonia, Shanghai and South Africa. (I haven't met them all yet, but no reason to believe why not.) I think this world is the biggest WOW there is, and I appreciate everyday that we have the means and ability to be awestruck by a sight, sound, scent, or experience nearly anyplace on earth.