It is rarely the extract most homebrew shops go through extract fast enough and the packaging is usually quite good. One of the things you are probably looking at is like denny said you are using tap water. The rule that if your tap water tastes fine is not really all that useful. Chlorine and chloramine have very mild tastes when used at home but can make horrific beer.
I am also not a fan of the brewers best kits personally. I don't carry them in my store because the grain is pre-crushed and can sit for months. Cracked grain is really only good for about 4 weeks before it goes stale.
You should try going with a homebrew shop that makes its own recipes. I do for all of my beers I sell and I have brewed every beer kit so that I know what they should taste like. Ask your LHBS to make you up a recipe of something they have their. Or you could buy from me www.diybrewing.com
. Sorry for the shameless plug but sometimes you have to plug yourself.