To start discussing the creator intent of the Death Knight class, it's also important to lay out the creator influences, and there is none more obvious than the runeblade Stormbringer from British sci-fi/fantasy writer Michael Moorcock's Elric of Melniboné stories. Even the Wikipedia page on Stormbringer mentions the nod in Warcraft:
"In the Warcraft mythos, Arthas (a prince who betrays his country and becomes undead, ending up looking rather like Elric) wields a soul-stealing runeblade named Frostmourne (the name is similar to [Stormbringer's sister sword] Mournblade). Also, the minor character, Highlord Mograine, carries the Ashbringer, an anti-undead blade whose name seems a clear reference to the Stormbringer. (The 'corrupted' version of this blade, which behaves more like the Stormbringer right down to the blade speaking to its wielder, was one of the strongest blades available to player characters of World of Warcraft for a significant length of time.)"
So whether you're familiar with Moorcock's books or not, it's pretty clear that the people at Blizzard are up on their classic fantasy, and I'd strongly recommend the Elric books to anyone looking to get a handle on the whole soul-stealing runeblade concept. Stormbringer and its effect on its wielder give one an excellent basis for WoW's Death Knights and their own runeblades. Death Knights are literally soulbound to their swords, often giving them names like Koltira Deathweaver's Byfrost, and they form the core of their power with the souls they consume. But they are also a dark, corrupting influence, and they ask a terrible price of their wielders. Elric's Stormbringer was as addictive as a drug, and it often took the lives and souls of his friends. Frostmourne took the soul of its own wielder, turning Arthas into into an undead slave of the Lich King.
Unfortunately most of the adoption of this into the game had to stop with NPCs because of game mechanics.
Personally, as a fan of Moorcock's books and an avid RPer, I have a pair of [Frostguard]s playing the role of Dariahn's twin runeblades, Reckoner and Revenant, and declare any actual useful weapons to be OOC. This sort of "RP armor" thing is pretty common on RP servers as you're lucky if you can get a PvE set that even matches, much less actually looks good, so I don't have any trouble getting people to go along with it. Naturally, the [Greatsword of the Ebon Blade] also makes an excellent RP runeblade, and you'll already have one by the time you get out of the starting area. I encourage anyone playing as a Death Knight to find a nice weapon to play the part, give it a name, and succumb to the hungry singing of your own black blade!