
The answer proved to be elusive. It's pretty hard to pin down what about it makes many of us not just enthusiastic, but emotional -- and mulling it over prompted lots of you to extend the question not only to Last Guardian, but to the rest of Ueda's oeuvre, ICO and Shadow of the Colossus, too.
Well, Let's Not Overthink Things
"The emotional response thing shouldn't be a surprise," commented John Scott Tynes. "It's a trailer, just like that for a movie, and trailers have long been crafted to elicit an emotional response." I agree with him -- in fact, the heavily-rehearsed and manipulatively-crafted marketing act behind not only E3 trailers, but pre-launch hype in general is something I keep complaining about.
And many of you join Penny Arcade in the humored prediction that neither adorable little boy nor adorable flying kitty-thing ("The Giant Man-Eating Eagle Toriko," says RedSwirl; Simurgh or Simargl, asserts Technomancer) can possibly survive until the end of the game -- indicating that lots of you are finding the emotionality a bit forced or predictable.
But I'm insisting on the analysis because we've all agreed for years that there's something special about these Ueda games, and maybe we can refine our wishes for other games if we can pin down what it is.
A Boy And His Alien

One primary issue many of you raised is how the game presents a relationship between a person and a monster. Fred Zeleny notes that using a giant animal instead of a human companion keeps it appealingly clear of "uncanny valley" territory.
JV points out the pairing is a familiar play on the "boy and his alien" trope that's touched us through the years: See also E.T, Neverending Story, most of the works of Hayao Miyazaki, and any other of about a million fantasy stories and comedic pairings that've seen a human bond with a non-human for friendship and adventure. Lewis Denby describes "an overflowing sense of childlike adventure in the face of something tremendously alien."
But even this theme is a variation on one that's still more common -- simpler than "boy and his alien" is "boy and his dog," a key nostalgia trigger and a simple part of childhood for many. Mike Schiller says the quiet, intuitive pairing between a kid and an animal as "protection and companion... [will] remind you of the one...thing, when you were a kid, that you could talk to and trust unconditionally."
Of course, ICO has a similar central theme -- Yorda may be in the shape of a young woman, but to call her a "human girl" sells her short. She's an ethereal, otherworldly creature, and much of the charm in the pairing and the much-lauded hand-holding relies on just how different she is from the little horned boy, from the fact they don't speak the same language to the charming height disparity between hero and "princess." And as for Colossus, I've heard many, many people say they loved that horse more than they've ever loved any other video game companion, be it man or cube.

These unlikely companionships seem to juxtapose nicely with the sort of environments Ueda tends to present: looming, expansive, preternatural and lonesome. They often feature detailed, subtle patterns and architecture that imply tribal or spiritual ruins; they make players wonder what kind of places they must once have been, enforcing the idea of abandonment and decay.
Exploring such intimidatingly solitary, sad spaces must only enhance the player's sense of attachment to a companion. Many of you agreed with commenter Mike Grove's assertion that "even when you're in a completely desolate place, you never really feel isolated."
Another element these games --and this trailer -- share is that all of the gameplay is very visual and very kinetic. Reflecting on his own childhood, Ueda has said he was "interested in things that moved." As with ICO and Colossus, Last Guardian seems to depend on the interaction of two living things with the environment and with one another, compelling the eye. Combine this with another key trait -- the absence of dialogue or overt narration -- and the player can't help but engage his or her imagination to fill in the blanks.
As commenter Mike Grove puts it: "It sort of transforms the player into a reader-figure in the best traditions of post-modern authors, giving them a degree of authorship without demanding that they use all of it. The Last Guardian trailer really embodied this sort of storytelling - we're given a few snapshots of a relationship and almost no explicit information about it."

Why They Work
There's always lot of blab and blah about player-directed narrative, the perils of enforcing authorship, storytelling via gameplay, and all kinds of haute concepts that writers and designers alike dither around with without ever really nailing it down.
But ICO and Colossus have been hauled out time and time again as scions of our medium because -- silently, of course -- they've hit key markers through just a few overt, but deceptively complex design choices that make player imagination the priority.
This is one reason Portal is also probably permanently on the cultural short list of Ideal Games, by the way. Although in a completely different way, it uses the environment to engage and encourage the player to imagine the story beyond the surface.
No Art From A Vacuum
And I'd like to point out that Portal was done with the contribution of people who would've been creative writers whether or not it was in games, and Ueda would have been an artist in some other medium if not this one, further nailing home the need for cross-disciplinary game designers. We've got way too many games designed by people whose primary hobby in life was Dungeons and Dragons or strategy board games -- yeah, yeah, I know, go ahead and add your obligatory comment defending the artistic value of these pursuits as if we aren't suffering from an excess of their influence.
Anyway. One more reason the Last Guardian trailer made us all so happy: It shows that this game has the key traits to suggest Team ICO can do it again. Hallelujah.
Thanks to everyone for your contributions, thoughtful comments and participation in the discussion! You guys are the best, and all of you are cleverer by far than one of me.
Ooh, bonus: Images are all via this high-res Last Guardian screenshot gallery from Offworld.