I'm a little mixed on this... while I think L4D is a great game and I love the hell out of it, I think it's too early for a sequel. I know I haven't quite finished exploring all there is to do with the original and I've logged 90+ hours according to my Steam stats page. I've got gold medals on all the Survival maps, but I still think there's plenty of value left in the game, and I just think it's too soon to be moving on.
But that being said, the sequel could provide more of the gameplay that made the first game so fun. It could do it bigger, and better. It's hard to tell from just a little two minute teaser. (hence why they're teasers.)

Already? Rather too soon, unless you are you-know-who.
Still not even played L4D1, and I only started on TF2 when the amount of content increased a lot.

Man, I never knew L4D blew so hard online. I assumed it wouldn't be great, but dang...
Up at school we played the hell out of it. Most fun I've ever had playing co-op on the Xbox (in the same room). It's been a long time since I've gotten into a game like that. Luckily I didn't pay for it though, and I don't think I would. The atmosphere was awesome, granted I haven't played many zombie games, but L4D really got me on edge, a lot. Witch Hunting is so incredibly intense.
I do admire Valve's work, and the Source Engine was quite an achievement, but I fear they may be going in a bad direction here. It is really early for a sequel, too, I feel like L4D1 just came out yesterday, then again, maybe Valve wanted to just start over, it looks like they'll be making some sweet improvements in the sequel, kinda like Windows 7 being rushed out after Vista...just on a much smaller scale...and not as bad.

I would rather have seen something from Half Life Episode 3. Left 4 Dead never interested me that much, primarily because I am more into single player than multiplayer. I have played the first game with bots though, and while it isn't exactly bad, there's just nothing there to hold my attention for long. I honestly don't give a damn about this sequel of theirs. Give me Half Life Episode 3 instead, and I'll be happy. If it is good.

On No Mercy finale, the camping point would be the radio room, inside the empty closet that's next to the weapons rack. Just keep smacking and gunning until tank comes, then throw a molotov and keep the tank busy until he dies. Repeat.
On Blood Harvest, camp at the red barn house with the ammo stash, then wait for tank to come, exploit stupid AI and run across the roof beams while gunning down tank.
On Dead Air finale, which is the easiest finale, just camp at the empty closet at the end of the map. Untouchable by specials. Wait for tank to come, flame him up with gasoline or molotov, run like hell around pillars until he dies.
I hardly play Death Toll till the finale, but last time we did it, we used the rock exploit.
The thing about the game is, it gets too predictable. It's an excellent co-op game, and I enjoyed it more than my Doom co-op matches, but FWIW, it loses its appeal once you learn that the game is very predictable. Content-wise, it's already lacking once you clock about 10 hours of game time. Technically, it's even more ass. I'm still getting network syncing issues, random disconnects etc, and so are others in my friends list. All I'm saying is, Valve and Turtle Rock should fix their shit first before doing a sequel.

Yeah, now I see what you're saying. Funny thing is, I never ran into these technical issues so I wouldn't know anything about them. That happens with plenty of games, though; people will encounter issues that others don't. I am excited for the new one, though, since every game Valve has worked on has ended up awesome. They're like the golden goose of gaming. I kinda wish Valve had revealed more about the next Half-Life or Portal game, though.

It's called 'Left 4 Dead 2', but that's just the name for the new free DLC they've promised to give us. I know it. Otherwise, it would mean Valve are band of greedy, shamele... Oh, yeah.

Lets hope so. I'm more then happy with the price I payed for the game and what content was included I had a ton of fun with it and got many hours out of it. I'm more worried about them just realeasing the SDK, it going to take people a while to make good maps and campaigns with it. If LFD2 comes out and isn't compatible it may be pointless to start making those maps.

Not excited about Half-Life 2 episodes either, but Portal...now that's different. So much potential to be unlocked, so full of awesomeness.

Portal 2 would be droolworthy by any means. Speaking of Valve, when are we going to be seeing a new Valve-made engine? Source has been around for quite some time now, and it's getting a little long in the tooth. I can see why former mods like Natural Selection are making their own engine or using the UT engine over the Source engine because it's... old.

Source is Modular. Compare it's first iteration to what you see now.
Or maybe I just have low standards...

There's always a limit to what you can keep upgrading it to.

Of course. But you can continue to rip out chunks until the new version contains no parts of the old.
Also, considering their Hardware survey results, a good portion are usually (have not checked in a while honestly!) in the low-mid range. Aiming for a high end market would be a mistake, unless they can scale back a lot. But why do that when you have a decent running engine right now?
I do expect, deep in the bowels of Valve, something shiney is in the works. But we will not be seeing it for a while.

I expect to see it when the new gen of consoles come out in a couple years time. That's my guess.

RPS has an update on L4D 2, if anyone wants to read
http://www.rockpapershotgun.com/2009/06/01/left-4-dead-2-exclusive-rps-preview/#more-12370
TBH the new features make me excited, but I sure ain't gonna buy the game at release anymore. Not until I can verify that the game is stable, and the price has dropped. I can only imagine buying it in November, then Left 4 Dead 3 coming out next January.
Some of the comments really sum up what I feel. I feel like one of the 2 million guinea pigs conned into testing Valve's latest tech demo "Left 4 Dead".
-added-
http://www.rockpapershotgun.com/2009/06/02/the-left-4-dead-2-trailer/#more-12457
Trailer here, it's not looking bad, but I wish they'd just do something less linear. While the premise of randomizing objects in your way, mixed with being forced to get out of your camping place, sounds like a very good idea, I don't know how well it'll sit in with the fact that "hey bros guess what YOUR MAP IS ESSENTIALLY STILL THE SAME LULZ!"