I agree that technique is key in avoiding that classic extract twang all too often associated with extract brews. Granted going all grain or even partial mash does give your brew greater malt complexity you just can't quite get with extract alone.
Experience plays a big part in the quality of your brew. Many all grain brewers started with extract kits and moved to all grain from there, bringing with them the experience of having endured a batch or two with that twang - myself included - before they improved their technique.
My first two brews were extract kits and not having had the experience to know fermentation temperatures were so critical; they both had that classic twang., and I know in both cases the ingredients were fresh.
Not too long after those first two batches (and one chest freezer and Johnson controller later) I had 6 or 7 more batches with no such twang, and they were made from extract and steeped grains or partial mashes, one of which was a pale ale that took gold at a regional competition.
Most of what you will taste in your brew is a culmination of quality ingredients and the effort you put into brewing it, and with some practice you will gain experience and your brews will just keep getting better and better, no matter what the brewing method.