Well, it's smaller, it also holds some unexpected surprises. The matte finish won't scratch or fingerprint like the original PS3 fat. The Slim is definitely noticeably lighter, but it's still kinda big.
Physical comparison vs. original PS3:
The shape is relatively unchanged, as you saw from the pics this morning, retaining the George Foreman grill top and the same ports on the back. We thought Sony shrunk it down by moving the PSU outside the body, like the Xbox 360, but no, it's still the same. There's no power brick.
Its power and eject touch buttons are changed to actual buttons (an improvement, I think), and the USB ports are shrunk down to 2. Also gone are any of the card readers. Because of the decreased size, the Slim is much less stable in vertical mode, which explains the $24 stand coming in September. You also can't twist the PS logo on the front to vertical orientation, like you could the PS3 fat.
My favorite change is definitely the matted finish, but the decreased vents might mean that the slim is quieter and/or cooler. We'll see once we start playing some games on it.
Audible comparison vs. original PS3:
We tested the two consoles with a Transformers 2 game. While the fans were pretty much the same between the two consoles, the disc drive is noticeably louder (but not a ton louder) on the PS3 Slim. Using a decibel-meter app on the iPhone, the Slim got somewhere between 25-33dB when playing the game, and the PS3 Fat got between 22-30dB. It's not a huge difference, but you should remember that decibels are logarithmic, so every 3 decibels equals about double the loudness, give or take. So if this thing is accurate—which it probably isn't, since it's just an iPhone app—that's about twice as loud.