Insanely Twisted Shadow Planet
I spoke too soon -- although the first couple "zones" (Organic and Ocean) give the impression that the whole game has a fairly consistent motif, this gets quickly and violently mixed up: I won't spoil it, but later zones play with perspective, use lighting as a real mechanic, and even have some bright colors. Shadow Planet's content is some of the best out there.
That being said, there really isn't much content. Including several dies-and-retries on the final boss, and scavenging the world map for all your missed items, this campaign is maybe five hours, six at the absolute most. There's an additional "Lantern Run" mode to play, but it's some kind of multiplayer schtick? and it seems to be based on what is easily the least fun part of the campaign.
There are some distressing bugs, too, most notably, sometimes the buttons don't fucking work. I can't think of how else to describe it. Not frequently enough that I would get really upset by it, but a few times throughout my campaign, I would be mashing on a weapon hotkey for solid seconds and it simply wouldn't select. Also, and this is a little less important, the campaign completion percentage in the pause menu doesn't really seem to work at all -- even after exploring the entire map, beating the final boss, and collecting all the hidden items, mine only said 96%; and I've read elsewhere that it actually fluctuates.
The ending is a bit of a disappointment, too, given the bombastic introduction; in comparison, the final cinematic is pretty underwhelming. And although the aforementioned introduction has some wicked-epic musical accompaniment, there's nothing else like it at any other point in the game. Granted, the sound effects do a great job of setting the atmosphere on their own, but the boss battles (or at least the final one) could have used some high-energy background tunes.
Ultimately, Insanely Twisted Shadow Planet is imperfect and criminally short. But, during its brief lifetime, it's also some of the most fun you'll ever have with a downloadable title. Most of the time, the gameplay mechanics just work splendidly; and the whole package has such a unique flavor, that it just seems great.
Better than: Limbo
Not as good as: Braid
I wonder: what's going to happen with the main menu's Downloadable Content tab
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