For Death Knight tanks, threat has been and is still a major concern at that moment. Neither is it an issue with low DPS nor it is an issue for AoE packs. It is huge issue when you play with other solid tanks and you realize Paladin and Druids can maintain 6-10k TPS (Paladin on the top end, Druids on the low) and when your DPS starts cranking out 4-6k DPS and you're stuck topping out at 5k TPS.
   By far, Death Knight has the lowest threat of all the tanks at the moment. Maybe it's enough for you to handle your DPS. It isn't for me. It makes a big difference when deciding who should tank an encounter. DK threat needs to be balanced with the other tanks. We don't need more, we're not asking to be OP, but it is a severe hindrance on our raid viability.
It is a problem for those of us tanking in raid guilds competing with great players. It is much lower then other classes. Druids and Warriors get as much of a benefit as we do. Just because our threat increases does not make it competitive. Raids need to be able to see either a benefit or equality (indifference) in Death Knight tanking compared to the other classes or they will consistently find themselves "off-tanking."
It seriously matters. DK threat needs to be looked at. I don't know if it's the threat modifier of Frost presence or whatever, but something needs to change.
All tanks are equal in threat (which we know is completely false, even if you leave DK's out of the equation) and DK players tanking for the first time in their lives thinking that they're doing good TPS but can't complete a heroic because the DPS they play with is so bad.
Take Warriors and Druid for example. Warriors can at least keep up in the OT position because they're generating rage the whole time and don't rely on revenge to build additional threat, same goes for druids not relying on reactive abilities to do their job. Keeping secondary aggro as a DK tank without getting hit is near impossible because you're eliminating our main source of threat, Rune Strike.
Tanking caster mobs is an issue, but something we have solo tanked and can manage to sustain a reasonable amount of threat. Again when it's just a solid casting mob it's a bit frustrating because we can't use our main ability and don't generate rune power from being hit to build that additional needed threat.
Main point is to not make Rune Strike mandatory as our only high threat generation. We're sure most other Death Knights will agree that Death and Decay isn't an option in being the only other way to generate significant threat. Otherwise when Ulduar comes out our threat generation is going to be a complete joke and all DPS classes will out threat us because they scale and we don't.
Death and Decay is like a Thunderclap DOT that uses half the rage the warrior could possibly gain in the next 10 seconds. To increase our threat generation without increasing our damage, the obvious answer is more 'high-threat' moves.
We don't know if this entire issue with DK threat being low, but it should help, especially if you ever die while raiding. Some of the guilds who are seeing this problem have very high DPS, which implies they have experienced the content and might not be dying that much.