Pros:

-alternate routes
-fun new gameplay mechanics
-great atmosphere
-great for exploring
-environments flow from chapter to chapter
-fun to play

Cons:

-hoards come a little too often, making exploring the map less useful
-the finale chapter
  -not actually a "boss battle"
  -pointless rafting sequence
  -survivors don't actually drown, so there's no point in staying on the raft
  -flat raft provides no cover from "boss's" thrown rocks, meaning it's easy to be knocked off the raft, and be downed or separated
  -no cover from hoard or supplies at the end to defend yourself en-route to helicopter
  -super-tank can't actually be fought

Conclusion:

I really liked this campaign... right up until the finale. The atmosphere of the campaign was dark and cool, and loved how it offered a quick way through the map, but with more hoard attacks, by blowing the barricades, or a slower, longer path for the explorers like me, that avoids alerting the hoard. It was actually really fun to play...

Then there was the finale... It started out okay, the mini-scavenger hunt for the explosives, but then it quickly went downhill from there... or down a river, as the case may be. The giant tank that you call a "boss battle," we never get to fight. In fact, it deters us from shooting at it, stating it's a waste of ammo. Instead, we have a slow trip down a river that doesn't even seem like it's moving, over what claims to be deep water. However, that didn't a seem to deter the AI, who fell off the raft immediately... and walked behind it  the whole time. When the water got deep, they would disappear under the surface, only to continue walking along the bottom with no problem. The super-tank we couldn't kill continued throwing rocks at us, so they fell behind when one went down, and I was left alone, because I didn't want to leave the raft. We got separated because of a bit of floating wood, and by the time the reached me at the end, the hoard had torn all four of us to bits. I tried to help protect them as we headed to the chopper, only for the common infected hoard to kill every single person on the team from sheer numbers. I was the only one to make it, and with a sliver of health. Not because I'm bad at the game, but because the pointless raft split us up and because of the sheer amount of infected. Because most of the map is water, the infected ALL spawn at the end, where it isn't, so you have a maps worth of infected surrounding you, with no cover.

I don't get me wrong, I loved the rest of the campaign... until the finale. I actually felt disappointed at the end, because I was having fun the whole way through.