Although I agree with this thread, there are a lot of issues with it.
For starters, people have mentioned concrete walls a lot. Agreed that it's not a hard skill and even a moron could build something simple, but we have to consider the logistics/time. I'm going to make the assumption that realistically you wouldn't even start to consider walls/pits until at least several weeks into the apocalypse. I think that this is reasonable. Even if I had a truck full of Diesel that got me to and from my local building merchant, I then have to mix the concrete, pour it etc (most likely by hand). Do I want to use the last bit of fuel on the generator? (assuming I have one). It would take a lot of people, a lot of time and a lot of effort/fuel/materials to build a 6ft High/2ft deep wall around a small home, nevermind anything substantial. If i'm not really close to a water source for the mix, this will also be a pain. A pit would be better as although it would take a long time without some kind of digger (have you ever tried clearing a large amount of soil/clay?!) The best defense would be the kind Woodbury had with aluminium sheeting and vehicles - although wouldn't protect well against human attacks.
Killing all walkers is a hard one, although beneficial it's tiring. As others have mentioned, it's risk vs reward. They always make an effort to kill all walkers in a particular area/building before scavenging, but even this must be tiring. The last thing you want to do is tire yourself out- this is when you get sloppy and get eaten! I do think they could have done a better job at the prison though, they always seemed to bunch around the same areas, they could have done with some spear-traps or something, or even putting aluminum siding on the fences so the walkers aren't drawn in at the sight of fresh human.
I do however agree with the clothes ones. I know the climate where it's set is humid and warm at times, but if I was scavenging, I'd want thick clothing to reduce injury. Why is nobody making make-shift armour?!
I think the smartest people we have seen so far have been the hospital group, they have water, power and although fucked up, a sense of order.