Himeji Castle: Maybe I just caught it on a really good day, but it was about a million times more beautiful than any of the pictures had prepared me for.
I will second Teotihuacan. When my family and I visited, we stayed overnight in the small village nearby, in a hotel that was close enough to walk to the site the next morning. As we approached, the sun was in our eyes, and for a second, as we turned a corner, we thought we saw a huge mountain or hill looming in front of us. As we got closer, we realized it was the Pyramid of the Sun. Just enormous.
Also Todaiji in Nara. It is the largest wooden structure in the world. I wasn’t prepared for just how big it was. If you have been in Japan for a while, I think it is especially surprising–you are used to seeing lovely elegant temples, but in general the Japanese did not go in for sheer bigness. Todaiji is one exception.
Things that were much more beautiful in real life than in pictures:
Da Vinci’s portrait of John the Baptist. Much more impressive to me than the Mona Lisa, just a short distance away.
The Miroku Bosatsu at Koryuji in Kyoto. Not an impressive picture, but extraordinary graceful in real life. And it is VERY old.
Glad you enjoyed it! Athens can be an underwhelming city (polluted, modern, riots etc), but the Acropolis is stunning.
I’ll add Victoria Falls. I took my partner to Zambia and we stayed with friends who have a luxury safari lodge. It’s lovely and my partner didn’t want to leave. I insisted that we go see the Falls (I’d seen them several times). As this involved a long local bus ride and then staying at a backpackers’ lodge my partner was even less keen to leave the lap of luxury. She kept saying, “It’s only a big waterfall, what’s the point?”.
I persuaded her in the end, and after a long, dusty ride we got there. As the Falls drop into a gorge you don’t actually see them until you are right on top of them, you only hear them. When we turned the last corner I waited for my partner to say something, but she just stood in silence. Thinking that she was wondering why we had bothered to come I turned to look at her - she had tears of joy running down her face.
We spent three days there, most of it just staring at the Falls.
(Sorry, that ended up longer than I intended!)
I agree with Sir T-Cups - the Med is beautiful.
One of my most precious memories is passing through the Strait of Messina. It was absolutely stunning, we had a pod of dolphins riding our bow wave, and the power cables across the Strait were still in place - and it looked like we were about to hit them.
The Panama Canal I found to be hugely impressive. It’s not so much that there’s any one instant of “Wow,” just the steady accumulation of the work needed to have created that passage.
Old Ironsides, USS Constitution, is another thing that I found very moving in person.
[Turns, looks out of window]
Yes, it certainly is. 
Another vote for the Taj Mahal. I never knew how exquisite it would be up close. It’s not just a big beautiful building. Every inch of it is perfectly decorated. I will never forget the jeweled flower inlays. They were utterly true to their flowers, but shining like luminous fruit in the sun, nevermind that they were all made of precious jewels.
I haven’t seen it in person but a good friend tells me that seeing the Space Shuttle taking off is mind boggling. I saw an IMAX of it once and asked him if it was close but he said no, the bass is so fierce you can hardly breathe!
The Arc de Triomphe was much, much larger in person than I expected.
I’ll have more later, but that was the one that came to mind immediately.
I’ll second Jackson Pollock paintings - they look like nothing in reproduction but transfix you in person.
I agree with jovan’s “essentially all art is better in person”, and I’ll add Leonardo’s (unfinished!) Adoration of the Magi, the winged victory that people rush past on the stairs to get to see the Mona Lisa, and Humayun’s tomb (the first of the Taj Mahal-type structures).
This is what I came to say. My wife and I were at the Musee d’Orsay, walked around a corner and came face to face with Whistler’s Mother. We were awestruck. We sat on the bench in front of it and just stared at it for 20 minutes. I spent 5 minutes just marvelling at the talent required to make the lace from her headscarf look so gauzy against the black of her dress.
Tropical undersea life. You can watch a lot of Jacques Cousteau, but strapping on a tank and actually seeing it up close and personal is a whole different experience, something I could do every day for the rest of my life.
Oh, yeah, tropical undersea stuff! I did one of those submarine things in Hawaii and it was incredible - all those, you know, aquarium fish.
I admit, though, I was pretty underwhelmed by David (I mean, it was nice, but I expected to be bowled over), and the Acropolis (same thing - if you’ve seen the picture, you’ve kind of seen the thing).
It didn’t seem so big until my friends and I decided to take the stairs up instead of the elevator. Talk about terrifying if you have a fear of heights! And the stairs only go up two levels, which is not even half-way up.
And it’s only 2/3 the size it was the first time it was built.
Another one in Japan: Sanjusangen-dō, a temple in Kyoto that house 1000 life-size statues of the Bodhisattva Kannon. Once, when I was at the Tokyo National Museum, one of the statues was on display. It was georgeous. However, it was only one of a thousand. Pictures don’t do justice to how much over-the-top Sanjusangen-dō really is.
Spending a few weeks at the bottom will change that opinion around. It takes several weeks to boat the entire thing.
The Vietnam War Memorial Wall. It’s huge and all those names.
The Sistine Chapel was, for me, simultaneously more and less impressive. More because of the colors and all the other artwork, less because the chapel is smaller than I had built it up in my head.
St. Peter’s Basilica is more impressive - the place is huge! Giant ceilings, everything everywhere is carved or painted or gilded, it was like walking into an artwork.
The Vietnam Memorial in Washington DC is more impressive in person. You start out walking along a sidewalk, and it begins to dip down a little, with a small wedge of polished black stone next to it. The stone grows in height and some names are carved in it. As you keep walking, the stone wall gets taller and taller and it’s above your head and the names become more than you can count, names upon names of the dead, and you look ahead and the wall just keeps going. It was unbelievably moving.
Van Gochs Sunflowers blew me away when I saw them for the first time,prints just dont cut it.
Gibraltar seen from the sea,the Indian Ocean from the air and Bermuda from the ground are all pretty incredible.
Also in Thailand you can be walking through a very normal neighbourhood and then turn the corner to see a huge reclining Buddha,incredibly impressive.
Also, with the Vietnam memorial, it’s so organic - the way people leave pictures and flowers and cigarettes and such. It’s very personal. And then you go to that new WWII monument and it’s just blah - it doesn’t touch you. (The new Korean War one is very human as well - I didn’t think it would be any good, but I liked it very much. It has a granite wall like Vietnam, with pictures, which I don’t think works, but the statues of the men are wonderful.)
Dinosaur fossils. When I was 4, I was absolutely in love with all things dinosaur. My aunt and uncle lived in DC, and we went up to visit them, and my parents promised me we could go to the Smithsonian Museum of Natural History and see the dinosaurs! I was so excited - it was all I talked about for WEEKS. Then we got there, and holy crap those dinosaurs are big. It scared the pants off me and I had to be removed from the building. My dad didn’t understand it - “But she said she wanted to go!” I think we eventually came back in through another entrance and saw the rest of the museum while quietly avoiding the Hall of the Giants. But they really are amazing - you don’t quite get the idea in your head, maybe, until you see the scale of the bones in person, that this was a real animal that was really that big, and really so “fanciful”. It’s very cool.
ETA - also, tigers up close. As a teenager I went to a zoo camp where you got to go behind the scenes and see the animals up close. Tigers are really, really big.
Returning the mention of NYC from the other direction / back at ya:
The night sky, as seen from places a long long way from the city lights. Especially high and dry country. No photo can possibly prepare you.
Excellent choice. And just imagine that this used to be a normal sight for us (the human race) and now most of us hardly ever see it. It’s something we’ve lost that I imagine was hugely influential on the collective psyche.
May I recommend lying back on top of a tall dune in the Namib Desert. Un-fucking-believable.