It's totally a trap. A propane tank explosion will stun a witch for a couple of seconds but it's definitely not going to one-shot her.
Right now I'm taking a break from trying to beat L4D2's first campaign's second level on Advanced in single player after like five unsuccessful attempts. Man these bots are dumb. We're at the beginning safe house. There are four medpacks. Two bots pick one. Another picks one, and uses it, so there's one left. I'm nearly at full health and I'm carring a box of explosive rounds (which take the place of a medpack!). Logically the bot who just used a pack should pick up the remaining one, but no, he won't.
The level is legitimately difficult though, so I can't blame it all on the AI. It's indoors, mostly big open areas with lots of clutter, and it's dark. Really dark. It feels much darker than any area in the first game.
In any case I'm probably going to start over on Normal. That, or stay on Advanced but pick up some human comrades, which seems the better idea.
In other news, holy shit the grenade launcher is nuts. The TF2 demoman is launching firecrackers by comparison. It feels like you're throwing propane tanks that detonate on impact. The friendly fire is kind of brutal though; I'm not sure it's going to be much use on Expert.
The chainsaw is crazy fun, it's the equivalent of putting up a wall of death in front of you, but its limited fuel introduces the idea that you might actually run out of offensive options completely. Though that's not so bad since melee weapons seem fairly common so far.
What else, what else... The laser sight is good, basically a straight upgrade item, but I find its best purpose is that it lets you see where everyone is pointing their guns.
The incendiary and explosive ammo don't seem like a huge deal so far, but every bit helps I suppose. Thinking back on it I'd rather take a medpack though.
Entertainingly enough, the first "oh shit!" moment happens seconds into the game. [spoiler]You pick up your medpack, you get your choice of extra pistol or melee weapon, and you get your... err... Uh-oh. Nope, no tier 1 weapons here![/spoiler]