Primarily, I hate Valve's new direction in shading for Left 4 Dead 2 (Yes, the SHADING is my biggest concern, I'm alright with everything else). You see, there are a few key similiarities of the shading in Left 4 Dead 2 (at least, from what we saw in the teaser) to Team Fortress 2. To highlight these concerns, I took a few screenshots and made a contrasting image.
[http://img192.imageshack.us/my.php?image=l4d2t.jpg]
You'll notice the very similar shading on their arms. And if you OWN a copy of TF2, I suggest you listen to Developer commentary on the map cp_well, there's a segment with the heavy that talks about how this particular type of shading is very key into making it
LOOK VERY CARTOONISH!!!
Really, I'm as excited as anyone for this game, I can't wait to start chainsawing up zombies or smashing their heads in with frying pans, but my biggest concern is how REAL it's going to look.. L4D took co-op survival
HORROR to a new level, because no other game looked this gritty, realistic, and terrifying the first time you step foot in it, and it continues to be surprising and fun-filled even so many months after we've all played four campaigns fifty times over, and it still remains terrifying when it's just you and a hunter, seeing who can kill who first. I hope this teaser was only done to excite our little nerdish glands in our brains, because other than that, it looks like it's not going to be "terror", more like "Legend of Zelda: Wind Waker" with more blood and zombies.
Edit: the way the shading makes it look cartoonish is how sharp it goes from light to dark, with no softness, which obviously looks extremely unrealistic.