When I see people, and particularly conservatives, discussing why one poll is better than another, I see the sample pool frequently cited as a reason for favoring one or another. Specifically, some poll watchers insist that any poll not filtered for likely voters, instead of just registered voters or even adults, is not useful in a political context.
The dirty secret is that not all likely voters are created equal. Every pollster has his own secret sauce, and we have to be careful when trusting that kind of filtering. It might not be what we expect.