Thank you to everyone who entered this challenge, you are definitely a skilled group and I appreciate the quality of entries.
I will post my comments in order of entry.
aaacb: This is a subtle but still very effective use of perspective. I am impressed with how the pillar in the foreground places the wall and fallen leaves some distance away in the background, the shrinking size of the leaves gives a sense of distance to the left and the monochrome treatment gives a sense of decay over time. Your use of perspective really adds to the mood of the image.
SpecialK: Your use of extreme perspective gives me a sense of how open (and bare) the plaza is and how high the surrounding buildings are, especially from the perspective of someone standing in front of the brown ball in the foreground. It also has some subtle features like the two people sitting on the left, with no one else around them except for the pigeons. It's not using perspective, but the contrasting colours and geometric shapes also add to the appeal.
pakinjapan: Great use of lines, on the street, in the lights of the shops, the contours of buildings, even the pedestrians look like they are walking in lines, because of how they are framed by lines on the street and the buildings, along with appearing to be far away. The use of the DA 10-17 makes the buildings curve to enclose the street and exaggerates the empty space in the foreground.
charliezap: Using a long telephoto gives the perspective of standing at the edge and looking down at millions of gallons of water in freefall, that is an awesome experience in person but is so hard to convey in a photograph. All those people below who are taking pictures won't be able to capture that, even though they can certainly feel it. It's interesting how you get a sense of being so close in a photograph that is taken so far away.
Vaskebjorn: Another example of having to get further away from the subject in order to make it look like it is very close. Your photo feels like we are almost nose to nose with your cat, thanks to the use of 75mm focal length and putting the cat up high by laying on the floor. Taking the picture this way makes it look like your cat is looking down his nose at the silly human with a camera.
atupdate: This picture makes me feel like I am about to take a few steps to the front of the stage and face a huge audience in the auditorium, capturing both the closeness of the stage area and the overwhelming size of the seating area. Even though the stage and auditorium are empty, it feels like something is about to happen. I have difficulty getting a sense of depth in an ultra wide focal length, but you nailed it.
jchorst: I really like how you have used the busy, colourful background to isolate the hands and wallet in the foreground, to give a strong 3-D sense to the entire image and providing context for the main subject. You also get a sense of thousands of wallets being on display, even though only the first few rows are in focus. This is an excellent example of using out of focus areas to provide perspective.
noelcmn: By offsetting your point of view a little from directly below the bell, you give a much better sense of the height of the bell tower and the position of the bell in the tower. By looking up from close instead of ahead from a distance, you give us a much different perspective and the rusty parts of the tower wouldn't be visible if the picture was taken from a different perspective. You could have taken this photo from a "normal" perspective, but the image would have been much less interesting.
sealonsf: It is interesting how you were able to get a sense of distance with a focal length of 18mm and f20, so basically maximizing your depth of field without getting a flat perspective. The angle of the sign in the foreground combined with a wide field of view is very effective at giving a 3-D perspective and combining it with the road heading to the vanishing point gives a sense of going somewhere, which isn't easy to do with a still photograph.
Hattifnatt: Perching the cow on a tree branch is a pretty neat trick, but I also like how you contrast very detailed elements like the tree in the foreground with fluffy clouds and indistinct trees in the background to give a sense of depth. Putting the closest elements on the far left also gives a sense of depth to the whole image. The use of perspective is very subtle, but the end result is a very appealing photograph. I think this is part of how you manage to take such good photographs of scenic vistas in general.
eva: You make a good argument for getting the DA 21 Ltd., it is very effective at giving a sense of looking up at the windows and buttresses of the buildings from street level and it really conveys a sense of old, narrow streets that can only be travelled on foot. We get a sense of the citadel on the hill in the background being far removed from the lower classes down below, partly through converging lines and proportional sizes, but also because the castle is in bright sunlight and the immediate foreground is in shadow.
What might be lacking in quantity is definitely made up in quality, but I have to pick first, second and third, so here goes:
First place: charliezap
Second place: atupdate
Third place: pakinjapan
Honorable mention to everyone else, thanks so much for participating.