Also keep in mind that the demo seemed to be the very beginning of the game, and you're just fighting an initial band of unarmed thug types. I'd guess that doesn't continue to be your main opponent for long.

I'm in love. High production values, controls are tight.
The combat is so fluid. He's just effortlessly doing combos and moves, and everything he does looks natural. His movements never looks like a pre-choreographed string(even though it is, but the variety in which he does it is like in no other game around, at all. The counter attacks are brilliantly contextual and you can chain your movements so easily it is amazing that you're doing it all with just a couple of buttons. It will be difficult to go back to combat in other action games now having experienced this.
This is the best stealth game ever. All other stealthers are clunky and frigid in comparison. Most stealth games are really slow. Here, you're able to move around and make decisions on instinct (rather than waiting the AI pattern) and the controls are so responsive which is key to everything in this game.
I love how nervous the peons get. Some guy was nervously scoping in every direction, a pipe released some steam, he went ballistic and started firing wildly. As soon as he calmed down from it, I struck!
I'm defo pre-ordering the Collector's Edition, and defo worth the wait for PC, which will blow my mind even more on the already impressively looking game.
The benchmark for an instinctive, responsive stealth and personifies Batman.

I knew I remembered the name Rocksteady from somewhere. I checked, and they're the people that made that really awful game Urban Chaos: Riot Response. I'm completely taken aback by this, since that is the ONLY game that they have ever released, and they move from that to this awesome Batman game.

I'm really looking forward to this, but I'd rather not pay $100 for the game and a Batarang that will just collect dust. The Modern Warfare 2 pre-order bonus made me think about actually investing in a nice, $300 pair of night vision goggles, and that's never a good sign...

Rumour has it that they're involved in Hitman 5 as well

http://www.kotaku.com.au/2009/08/batman-studio-rocksteady-named-in-hitman-5-development/
Not taking it over but apparently being involved in some capacity

What I really want to know is how a studio who has only produced one very mediocre game is being given all these massive, massive properties to roll with.

Well, there's certainly precedent in the media industry. Have you seen any of Peter Jackson's movies from before Lord Of The Rings? You have got to sort of admire the testicular fortitude of the movie exec who decided to give him a bazillion dollars for making what is arguably the most anticipated book-to-film conversion ever.
Don't get me wrong, I'm a big fan of his older movies, particularly Braindead, but I never understood why he was the one to make LOTR.

Its not the most logical jump, from incredibly low budget new zealand alien/zombie b movies to one of the most inexplicably bleloved properties in all literature but hey, he probably put together a good pitch
As above, the pitch was probably what sold it. They're probably rabid fans of the Batman universe and wrote the best damn pitch out of anyone and someone had the balls to actually take a leap of faith for once rather than sticking to the predictable and safe.

Urban Chaos: Riot Response was genius. It wasn't a very good game, but it was a great statement about games, and was incredibly fun. The game turned out as well as its concept could possibly allow.
But Aliasalpha's right - pitch is everything. It's what I love about this industry, underdogs can make some really big breaks if they play their cards right. And its with those big breaks that they're more likely to make an excellent game, since they don't have the complacency of veteran developers.