You just have to treat extract carefully. There are 2 things that I think cause "extract twang". They are Miallard Reactions and caramelization. These can lead to flavors such as bready, biscuty, caramel, and buttery.
Miallard reactions are non enzymatic browning reactions that happen between an amino acid and sugar. They are very complex, and not all of them have been documented. Maillard reactions are responsible for toast, browning of meat, and malted barley. Some Maillard reactions can happen at room temperature. This is why its important to buy fresh LME. The longer LME sits at room temp, or warmer temperatures, the more suseptible it is to these reactions.
The other is caramelization, which occurs between two sugars. Caramelization can effect extract brewers in two ways. First is if you dump LME into the pot without turning off the burner. The hot bottom will scortch the concentrated LME. Extract brewers also tend to do smaller boils. While an AG brewer is boiling 6+ gallons for a 5g batch, an extract brewer might be boiling 3 and topping off. A concentrated boil a concentrated boil can increase chances of kettle caramelization. It also can increase melanoidin production (a Miallard reaction).
Extract brewing is not necessarily inferior to All Grain. All grain gives you more control over color, fermenability and freshness. If done correctly, extract can produce some great beer. Stepping up to full boils, using fresh extract and mixing in the extract before putting back on the burner will assure the best quality extract beer.