IIRC the Death Knight template in DLcs is not a replacement for the one in MMII, it is a supplement to it, adding more abilities to the top end for high HD death knights.
Most importantly, it adds power word kill, which is for some unaccountable reason missing from the MMII version.
The one I used a few sessions back used the rules as written, with DR/magic and Power Word Kill. I also gave it the Unholy Toughness special quality, which let it use its charisma modifer as a HP modifier.
I'll never understand why death knights get wall of ice as a special ability. I mean, power word kill, fine. Abyssal blast/fireball, fine. But conjuring a bunch of ice? WTF?
I used a death knight's wall of ice to great effect a few sessions ago, when the dk separated the party's fighter from the rest of the group and then raised the wall around the two of them. The dk eventually lost to a summoned huge earth elemental and a ghaele eladrin.
Lord Soth used Wall of Ice quite.. Differently.. in one of the novels. Used it to touch and freeze a locked door, then kicked the door down and went through.
My understanding is that DL Death Knight has power words, wall of ice and also can create Skeletal Warrior that I believe do count towards his undead followers. I don't think there's much else different other than that.
Lord Soth used Wall of Ice quite.. Differently.. in one of the novels. Used it to touch and freeze a locked door, then kicked the door down and went through.
This happened in Test of the Twins, IIRC. However, it was also displayed in Knight of the Black Rose, where Soth seems to have several other powers never displayed before or since. James Lowder, the author of that novel, has explained that it was due to a lack of knowledge on his part at the time about Soth's exact powers (apparently when he was writing it details were harder to come by).