I've posted my melodic-death-metal dissertation here before, but that was 4 years ago, so let's give it another go.
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To me, the main characteristic of MDM (besides it being "metal" and the dominant vocal being growled) is a constant lead guitar melody laid over the top and winding and weaving throughout an entire song.
So I disagree with those who say that removing the growls would make MDM just "metal". In most normal metal, any lead guitar present usually drops out when the vocal comes in, allowing the vocal to provide the main hook of the song. In MDM, it's the guitar that has the sole responsibility of providing the hooks, and it usually supports the vocal, even giving the illusion that the growls contain melody of their own.
By this definition, In Flames's "Subterranean" is almost a perfect example of MDM, while "Slaughter of the Soul" barely qualifies at all.
In Flames is a good case study. Even by "The Jester Race", the MDM can be seen to fade a bit. Take the verses of "Graveland" and the title track, for example. The flowing lead guitar has been sacrificed in favor of the chunky riff. The riffs themselves *do* still have a lead guitar component riding on the top, but it's played staccato to highlight the jerky rhythm, in a very "Dark Tranquillity" kind of way. It becomes even more pronounced when we get to "Food For the Gods" and "Morphing Into Primal" on "Whoracle" (contrast them with "Jotun"), and by the time "Reroute to Remain" comes along, they don't even bother to include the staccato lead in the riffs anymore, apparently preferring to reserve all the sonic space for the bass-heavy chunk.
So really, some of the purest examples of MDM come from some of the lesser-known bands, such as Without Grief and Sacrilege. WG's "Deflower" is pure lead-guitar heaven, and I think the main reason that the follow-up album isn't as good is because the lead-guitar aspect was reduced. Even a relatively new band from the wrong country, like Searing Meadow, can make a purer (and really good) MDM album with "Corroding From the Inside".
And then there are the classic albums from bands that haven't traditionally been called MDM in the narrow sense, but they do the lead-guitar thing better than almost anyone from Gothenburg: Amorphis "Tales From the Thousand Lakes", Orphaned Land "Sahara" (and even "El Norra Alila"), and Edge of Sanity "Crimson".
My favorite Withering Surface album is "Walking on Phantom Ice", but now I realize that it's not an MDM album at all, despite the fact that the band's two previous albums clearly are. The melody comes mostly from the vocals and keyboards rather than any guitar lines. This is the same reason new Soilwork doesn't qualify under the MDM definition, and old Soilwork is disqualified for the same reason as Arch Enemy: the lead guitar parts are too compartmentalized and separated from the "thrash" parts, either in dedicated "solos", or too-obvious hooks.
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http://www.youtube.com/watch?v=iCNPFVJGiMc
http://www.youtube.com/watch?v=Whu2_5O51lw
http://www.youtube.com/watch?v=XJSNfZNJD-g (crappy YouTube upload that doesn't let you hear the awesome sound of this record)