For styles that are heavy on specialty grain and/or hops, a well-made extract beer would be very hard to distinguish from an all-grain version in a blind tasting, IMO. From a quality standpoint, if you pay close attention to fermentation and sanitation, and use the freshest extract you can get your hands on (for LME especially), then your extract beers will be damn good.
There are some styles that you would have a real difficult time brewing with extract. Dunkel is the first example that comes to mind, since I know of no Dark Munich extracts out there. Any base-malt forward style is going to be tough to brew using extract. But IPA's, APA's, Stouts, Porters and so on will come pretty close to all-grain versions if handled correctly. I still brew almost half of my IPA's using extract, and all of my 1-gallon test batches of APA for sampling new hops.