L4D2: The Passing is an add-on, detailing the crossing paths of both games' survivors (sans 1 person, but I'M not giving it away). There are 3 maps to this add-on: getting through the city, crossing under the river, and getting the generator to start to get the bridge going.
New additions/the good: hooray for more dialogue! If one thing Valve does correctly is interesting and humourous dialogue between characters: Ellis hits on Zoey, Rochelle offering Francis a kiss for lowering the bridge, or Louis forgetting Nick's name left me rolling in my chair.
The special infected with all the loot and med kits seem to be able to outrun a typical zombie and take a buttload of more bullets, but I guess they had to make it a challenge somehow.
The so-so: it's still the same game. Nothing new is added, except to story. And, well, a golf club.
The annoying: mutations. I have no problem with the game mode itself; it adds a new and different touch as to how a campaign should be completed. What I DO have a problem with is the graphics clipping/engine problem. Say I'm carrying a gnome through the first chapter and I get to a bridge with a chainlink fence on either side. Obviously, an area I can not get into through the fence. Then if I put down the gnome to defend myself and throw a grenade, why should the explosion send the gnome itself THROUGH the fence and render it unobtainable? Forcing me to start the chapter over simply because of the engine problems?
A neat little story for just 7 bucks (560 MS) with a few added challenges.