These items have probably already been mentioned, but just thought I'd pass them along...
From Treehouse Live, Day 2, with the developers:
- There are four maps currently shown in the trailer and demo, but "many more" are in development
- The level designers have so much fun making maps that people in the department next door keep telling them to calm down
- Some maps will focus on wall-climbing; others will emphasize varying elevations so you have to consider the angle of your shots
- Squids were chosen over octopi because their arrow-shaped heads make it easy to tell where they're going on the GamePad map
- Likewise, shooting ink instead of bullets is useful since you can see the attack before you're hit
- The art direction is final, with the more urban environments meant to make you feel like you're spreading graffiti
- The music direction is also final, and the idea is the music heard in the trailer and demo is music the Inklings listen to in their world
- To go along with the above point, this is why some of the Inklings wear headphones
- Members of the dev team are the ones yelling "Splatoon!" in the theme song
- The ink-spraying competition is a fun sport for the Inklings, and the ink is an eco-friendly substance that disappears with time
- The sound effects team worked really hard on the sound of the ink splattering in various ways
- The texture of the ink changes based on whether it's a fresh coat or multiple layers
- They promise the graphics will continue to improve more and more leading up to launch (they already look fantastic, IMO)
- You pick your loadout before each match, I.E. a charge-shot sniper rifle, a paint-roller melee weapon, etc.
- You might be able to customize your character's appearance, as well, but they're still thinking about this
- As others said previously, there will be a variety of modes offering a lot of content (including single-player)
There may be more, but I'm going off of memory, having just watched the half-hour segment.