I dont think Valve has put a lot of thought into survivors pre-firing corners/walls in combination with the infected's propensity to give away their position in the official campaign maps. Or conversely they put a lot of thought into it and wanted an "easier" game on the survivors to attract customers and they slowly toughen it up after the game was released. Believe me, my partner and I are taking this stuff into account. We are making sure we have more walls survivors cant shoot through, thus making more good ambush points for infected to attack from.

I agree, I think that the first two official versus maps are kind of set up to allow for a learning curve and that newer maps will be much more challenging for survivors. I also think that the game is just not that much fun when you can incapacitate the survivors too easily, I love to beat a team quickly but it is even more fun when you harass them for a whole level and manage to stop them just in front of the safe room.

As far as AVS' comment about a lack of good ambush spots, I think there are plenty you simply have to take certain things into consideration. If you are playing with a team you are familiar with or have had success in getting them to coordinate then your options for ambushes greatly increase. I do find it fairly difficult to play with a team that isn't talking as they tend to be newer to the game and thus it is a bit more difficult to anticipate how your actions might best benefit your teammates.

Here is more of my 2 cents:

When looking for a good ambush you need to consider the most crucial (IMO) infected on your team, the Boomer. A good boom instantly elevates any ambush's effectiveness. What constitutes a good boom is dependent on a given situation, but if you can get at least 2 survivors you have a formula for success. I think you can make any area into a good ambush so long as you can get at least 2 survivors boomed, a decently long smoker pull, and hunter pounces on either non-boomed survivors or any survivor that has been good at rescuing while boomed. Whether or not a good ambush becomes a game winner depends on the strength of your ambush positions and/or a little luck. I think what some people are looking for in this thread are typical game winner scenarios which is perfectly fine, you want to know about these to use and defend against them, but at the same time I think it is more important to consider the aspects of a good ambush so that you can come up with your own. In this way, you are not giving a man a fish, but teaching him how to fish.

If you've read to here, then have a fish:
Not sure if this is widely known or practiced around here, but I just watched an awesome smoker tactic on NM5 the other day. Everyone knows how devastating it is to get trapped alone at the rooftop entrance with a hunter and your teammates have already jumped down. But I watched a smoker on my team set up a great pull at the rooftop entrance that had nothing to do with the satellite dish: instead of setting your smoker there, you hide him behind that big metal pole to the rear of the roof. There is a ventilation system just off of that which the smoker may be able to use to hide (I'm not sure since I have not had a chance to practice it), or you can simply stand behind the large pole. Once at least 2 people are off of the roof, you move out and snare the last one off and pull them back into the corner. The pole is great because you can spawn there if you choose, it provides some cover from fire, and there is no real way for survivors to free their teammate unless the move out along the ledge of the helicopter landing pad.