The first concern when it comes to Death Knight tanks is we don't really know what mechanic they'll use for physical damage mitigation. They don't use shields, so that gives them a hit to their armor and guts their ability to block. With the current Crushing Blows mechanic, this puts Death Knights at a severe disadvantage. One of their self-buffs, their presences, is an ability that increases their threat output and their armor value.
Here's my theory: The armor increase from their Frost Presence won't push them into Dire Bear levels. It will compensate for the loss of a shield. Blizzard has stated some distaste of their Crushing Blow mechanic, and has experimented not using it at all on some boss encounters. We may see a removal of Crushing entirely, with Frost Presence putting Death Knights roughly on the physical mitigation level of Protection Warriors and Protection Paladins. Death Knights will probably have scaling issues on this front unless Frost Presence scales with gear. If it doesn't, they may fall behind Warriors and Paladins(with Druids leading the pack by far) in the end game, leaving the other classes to the physical stuff, and Death Knights holding their niche of the magical stuff.
A lot of Druids were up in arms about Death Knights taking their 'niche' of magic-based encounters. I will admit, I don't know how widespread this concern is, but I heard it quite a lot in the first few days. They confused me, but what can you do? Personally, I don't think Druids will be bothered at all by the coming of the Death Knights. In fact, they will be tanking two completely different things. Death Knights won't be taking the tanking jobs of any of the traditional tank classes as their main tanking role.
Mage and Warlock tanks will be the ones set aside. Yes, Mages and Warlocks. High King Maulgar, Leotheras the Blind, Illidari Council and Illidan Stormrage utilize these classes for tanking magic damage. The gimmick of Spellsteal is reborn in the form of the Death Knight's Anti-Magic Shield. Consuming a rune(or runes) to give 75% magic damage reduction at opportune moments won't be far off from Spellstealing a boss's super powered Dampen Magic.
Warlocks are good magic damage tanking candidates for a number of reasons. Searing Pain, high Stamina, and Fel Armor gives them excellent threat and they're easier to heal than most other options. Death Knights, being a tanking class, most assuredly will not have trouble on the Stamina or Threat fronts. They probably won't have Fel Armor, but their other natural defenses will probably make up for it quite well.
It is understandable to be concerned about people moving in on your niche. It isn't pleasant to think about your role being filled by someone else while you stand back and watch. I don't think this will be the case for Death Knights. They are tanks, yes, but their primary role takes over the Mage and Warlock tanks. This is a good thing. Mages and Warlocks will remain useful with the addition of Death Knights. Min-maxing aside, Mages and Warlocks will still be taken to raids, even if they don't tank things. Not tanking won't remove their niche.
It gives Death Knights their own, personal job to do. They will be capable of tanking physical damage like Prot Paladins, Prot Warriors and Feral Druids, but I doubt they will be as good at it. The other tanks will keep their job. They each have their niche. Death Knights simply fill a niche none of those three did. They fill a role that was previously a gimmick, and will probably come into play quite a bit more to make taking a DK to a raid worthwhile.
When it comes to 10 and 25 man raids, sure, it probably seems like someone will be losing their raid spot, but there will inevitably be people rerolling specifically to play this class for your raid, or people coming and going with the launch of Wrath for whatever reason. Things will be okay. Really.
To summarize: The role of the Death Knight doesn't step on the toes of Warriors, Paladins or Druids. They'll do their own thing. It's scary making room for the new kid, but things will be alright. Let's wait and see.