Yeah the longer the show goes on, the more far-fetched it's going to seem that there are still so many zombies roaming around. You'd think most of them in the area would have either been killed off or wandered off somewhere else by now.
Actually the reason that there are so many zobies wandering around is because survivors keep making noise.
In the comic they explain this pretty clearly. Roamers will keep moving in the direction of noise until something new attracts their attention. Given that the world is ALOT quieter post apocalypse, sound will travel much further and attract more Zs than would normally be the case. Everytime the survivors fire an unsilenced bullet they attract more Zombies. Before long you've got a herd. The noise from Daryl's chopper would have added to the problem.
Noise is also why settlements inevitably attract roamers. Civilization means noise, which will in the long run produce a herd. I think that it would have been better for the show to have pointed out that all of that target practice and wasting bullets early in season 2 did more to attract the herd to the farm than the helicopter and that final bullet that killed Shane.
On the weather, in the comics the first winter was supposed to be VERY cold. Its not stated why, but the book World War Z gives a plausable explanation in that all of the fires started as civilization fell produced enough smoke to block out enough sunlight to bring down global temperatures (sort of like a Nuclear Autumn/Winter). Consequently, things should have been easier on the surviors during the winter. The Zs would not die but they should slowdown and possible freeze if it got cold enough. Finding places that are warm, zombie free and have access to sufficient food and clean water is what should have been the biggest challenge.