I was reading through Darryl Richman's book on Bock beers in the Classic Beer Style Series. He talks about some studies regarding the differences in chemistry between LME worts and all grain worts. DME was not included.
To sum it up, he says there are three problems with LME: potential oxidization during handling and storing, low Free Amino Nitrogen (FAN) content, and adulteration with sugar syrups.
By "adulteration" he isn't stating that the makes of LME are cutting LME with glucose syrups. But "somehow" the percent of glucose in some LMEs are far greater than they should be.
If the LME is packaged with oxygen, the melanoidins (associated with darkness of wort and malty taste) can become oxidized and cause off flavors. The longer the LME is stored, the more oxidization can take place and more off flavors produced.
LMEs are typically low in FAN. Some contain between 1/2 and 1/3 the amount of FAN that yeast needs in a wort for a healthy fermentation.
LMEs may become adulterated with glucose syrups. This both dilutes the FAN content of the LME and creates in imbalance in the sugar balance. For a normal all grain Bock, 25% of sugars are glucose and 45% are maltose & maltotriose. A LME that is adulterated with glucose syrups could easily have these percentages reversed.
This means there are too many simple sugars and too few complex sugars. This prevents yeasts from having high reproduction rates early in the fermentation which increases the potential for long disordered fermentations and wild yeast to gain a beachhead in the wort.
One possible reason why DME may not be exhibiting the "twang" is because it is made in a partial vacuum. This means that there is less potential for oxidization of the DME.
Darryl Richman suggests two possible ways for eliminating the problems associated with brewing with LME. 1) Use the freshest LME possible. This way any potential oxidization is reduced. 2) Partial mash. This helps to restore the FAN content of the wort to what it should be. It also introduces more complex sugars to the wort if the LME was adulterated with glucose syrup.
As for me, I've never noticed the "twang". But I always partial mash, even if its just 8oz. of Carapils for mouthfeel. The only time I have been disappointed in my use of LME was the one time I used Munton's LME. The rest of the time I've used Coopers and I haven't had any problems.