I'm surprised that Summer Wars is so well-received. While enjoyable to watch, the whole movie has such an artificial, romantic-comedy-esque, "we can do it" spirit. I think I might have actually thrown up not ten minutes into it, but I'll try to break down what there is to like about it.
(Warning: Spoilers all over the place)
STORY: The movie opens with a description of Oz, which is your standard virtual world where you can live a second life, except people use it for important things as well. I was hoping there's more to it, but that's pretty much it. Then break to Natsuki who urgently needs one of two nerds to go with her on a weekend trip to visit her family. Kenji goes with her, annddd...surprise! He's forced to pretend to be her fiancé in front of her overbearing family. Yeah, that's where this goes. And despite some bumps and hurdles, the Kenji-Natsuki relationship is secured in a matter of a couple days. Not much to see here.
The other part of the story is a tad more interesting but equally lacking in depth. Kenji gets a code in a message on his phone. He cracks the code and discovers the next day that he broke Oz's security encryption and that Oz has been taken over by a malicious AI (if you're familiar with cryptography, you should realize this is not at all plausible). But wait! Turns out one guy has a supercomputer and this other kid is the strongest fighter in the game and another guy can just procure a vehicle from the military for portable internet. Good grief. Talk about contrived. And they save the world by playing videogames. It's like watching Pokemon for two hours (Pokemon might actually have a better story).
Despite that, there are a few compelling moments, and I'll admit the pacing was even enough to keep me interested. The death of the grandmother served as a proximate example of the consequences of Oz being hacked into, and makes you reconisder depending on technology so much. The moral question of Wabisuke's responsibility for the actions of his AI, while not original, was well-framed. Still, most everything that would have made the story better was squashed under an avalanche of horse-beating jokes and what you might call "motivational perseverance" sequences. Every few minutes we get to hear (1) someone crying, (2) an inspirational speech, and (3) everyone becoming inspired and the mood totally changes. And then back to (1) again.
CHARACTERS: The characters are no better than the story. There is plenty of variety, but they're so archetypal. Kenji is your stereotypical, girl-shy math whiz. Natsuki is (gasp!) the most popular girl in school. Kazuma is a videogame-playing shut-in who was bullied when he was young. Perhaps the only character with any originality and depth is Wabisuke, an adopted kid who went to America to write an AI that hacks computers with the intention of selling it and bringing wealth back to the family.
Over the course of the movie, there's basically no character development. Really, there can't be any character development because we don't get to know anyone beyond their obvious stereotype. We hear breifly about Kenji not having a family, and we get to see the kind of woman the grandmother was. But besides that, the main characters are drowned out by too many minor characters.
ANIMATION/SOUND: Much better on this front. The virtual world of Oz provides brilliant, colorful images giving you an otherworldly sense that I would compare to a Miyazaki film. The virtual world Oz is where anything can come to life: the weak Kozuma is the strong and cool King Kazma in Oz; Love Machine takes various different forms as it evolves; an old game becomes a tool for saving lives in Oz. It's the animation that drives this. In the real world we see blander colors, painted backgrounds, and an obvious less "digital" look. In Oz, we see a vivid palette, red outlines, and everything is placed in a sort of endless void. It works really well. Besides that, the action and the animation as a whole is super smooth, and there's really quite a lot happening on the screen at any given time.
One minor complaint I have is the lack of character detail. Compared to the richness of the environment, characters have virtually zero shading (even in Oz) and they look flat and drab. This is probably to compensate for the number of things happening at any given time, so you gain a bit here and lose a bit there.
Likewise, the sound is equally high quality with chimes and electronics to match the digital world and more classical pieces to match the analog world. It was fitting but not something as distinctive as, say, Ghost in the Shell or Cowboy Bebop.
CONCLUSION: Summer Wars is a sensory experience. It's a movie I might want to see in 3D at a theatre. There was enough to keep my attention, but it's held back by clichés, generally uninteresting characters, and a lack of depth in the plot.
After writing this review, I've read some of the others. One thing I haven't touched on is the family dynamics. There are three or four generations represented and there's incredible variety in everyone's personalities, and I think this is all managed well. The only thing is you have to first accept each character's stereotype as believable.
Another thing people seem to like is how Summer Wars embraces the internet/technology. I agree, but this is nothing unique. There's Ghost in the Shell or the .hack series, for example, among others. Not to mention how a ton of anime centers around a computer-savvy nerd/geek type who uses the internet as a social outlet.