I read through all the posts here and have a few comments.
I don't think there is any single thing or procedure that causes this German Lager flavor. I believe it is a combination of things and simply a product how Germans brew beer. However, German brewing practices today are very much different from what we think. Most beer is mass produced, especially the big brands like Bittburger, Warsteiner, Radeberger ... Even most of the local brands are produced the same way. Most breweries in Germany are either owned by InBev or the Radeberger Gruppe, which is is part of Dr Oetker, a conglomerate that started out as a baking powder company.
A few month back I got pointed to a very good German TV documentary that highlighted how German beer had lost its way. For posterity, here is the link (http://www.zdf.de/ZDFmediathek/beitrag/video/1656374/ZDFzoom-Hopfen-und-Malz-verloren!#/beitrag/video/1656374/ZDFzoom-Hopfen-und-Malz-verloren
) there are a few German speakers on this forum for which that might be useful.
I just came back from a trip through the northern part of Germany and I have to admit that most beers in Germany do taste rather bland these days.
Beers brewed in southern Germany tend to be better. For one, they have a better beer culture down there and there is also a bit more variety. Especially when it comes to Weissbier.
As for the brewing processes that can give this characteristic German flavor, I don't think that decoction has anything to do with it. Most German beers are made w/o decoction and I have made many decocted beers that I would say have that German taste. This is not to say that decoction doesn't make a difference, lets leave this for a different discussion.
Warm vs. cold maturation rest? A few times I have tried the cold maturation rest, i.e. where the beer is slowly cooled after primary fermentation is done. The biggest problem was that I ended up stalling fermentation and the beer did not attenuate as expected. These days I often raise the maturation rest temp to 70 F for a week in order to make sure the beer fully attenuates before cold conditioning. The complete fermentation is more important than doing a cold maturation (a.k.a. diacetyl) rest. When I say complete fermentation I mean getting close to the attenuation from the FFT, especially for a Pilsner. A Schwarzbier can be a few attenuation percentage points off and a Doppebock can show 4%-6% lower attenuation than its FFT. Simply judging complete fermentation by the absence of activity may not be accurate enough. The low fermentable sugar content, that you get when the beer ferments to the attenuation of its FFT, is important to get a very drinkable beer.
One major factor in getting the German flavor is the aroma of the beer. German beers have a very subtle aroma. These days many beers don't seem to have any aroma since brewers skimp on good aroma hops. But even the good examples don't have a strong hop aroma. If hops dominate the aroma, its never like sticking your nose in a bag of hops. Instead the hop aroma is more refined. This comes from the fact that German brewers don't add hops late. Even aroma hops are added with 10-15 boil time to go. According to a number of sources I have come across the hop aroma compounds oxidize in the boiling wort and create less volatile compounds. This leads me directly to FWH. FWH doesn't appear to be common in German brewing, but I have had great success with getting that German hop profile into my Pilsner and Helles by using FWH.
Another important part is smooth bitterness and for that I strongly believe that the Kraeusen should not be allowed to fall back into the beer. I make sure that it blows off. Even if that means I have to add more sanitized wort after I realized that I didn't fill my carboy enough to get a blow-off. When I don't do this the bitterness gets a harsh character.
I also started experimenting with CO2 hop extract. The gooey hop resin that NB sells as the Hopshot. A few years back I bought a 150g can and put it into syringes. I know that the use of hop extract is frowned upon among beer geeks, but hop extract is able to add bitterness w/o bringing vegetal matter into the boil that can lead to increased tannins. I have not done a side-by side between a classic bitterning hop like Magnum and hop extract.
While I do have a number of thoughts on this topic, I haven't found that perfect procedure that will always give you the authentic German taste. There are too many variables that would have to be evaluated. Just be open to experimentation and start trying things that you haven't tried yet. This obviously implies that you can brew a clean beer repeatedly and that you brewing process is not plagued by more straight forward issues.