Rune regeneration can't wait
If you read my column every week, you know I'm not a huge fan of the current rune regeneration system. That said, it looks like it's here to stay, and it is currently designed such that it is mandatory not just for higher DPS, but to make a death knight's rotation smoother and manageable. This is why it's a little confusing that it's stuck at level 75. Try it some time. Respec, but don't assign yourself a level 75 talent, then go to a training dummy and see what it does with your rotation. You'll be faced with long dry spells where all you have available is auto attack and a small handful of free cooldowns.
New death knights have to put up with this for 20 levels. Not only is it annoying to have nothing to do but auto attack, it prevents you from learning your class rotation and leaves with you no innate survival options should you get in trouble other than the classic run away and hope they lose aggro before you die option.
Whether we keep the same rune regeneration setup in 6.0, or get an entirely new one, it very obviously needs to be granted as close to 55 as possible. Assuming we keep the current talent tier, my suggestion would be to swap the level 75 and level 57 tiers. Right now, the level 57 tier is most useful for raiding. Anti-Magic Zone, in fact, is being redesigned in patch 5.4 to be an even stronger raid cooldown to the arguable detriment of its use in PvP and lower end PvE. Losing the Lichborne self-heal trick would be a bit of a disadvantage for leveling, admittedly, but in leveling, you shouldn't need much more than Death Strike and Death Siphon, and it would be worth it for not having those long auto-attack-only droughts.
The bear necessities
When talking about leveling specs for the death knight, unholy is last on a lot of peoples' lists. Part of this is because it has a counter-intuitive rune-spending scheme that disallows the easy use of Death Strike, but another part is that you don't even get the full outlay of that scheme until you're well into the leveling process. You need to wait until level 58 to get Scourge Strike and level 62 to get Festering Strike. Essentially, that leaves you 4-7 levels in which you are either leaving behind lots of orphaned frost runes as you try to use Scourge Strike or are rocking the blood damage rotation with Death Strike. In order to allow aspiring unholy death knights to get into the groove of their rotation as soon as possible, both Festering Strike and Scourge Strike definitely need to be available at level 55.
With the other two specs, it isn't quite as bad. With Death Strike at level 56, blood has its rotation almost right away, and while frost doesn't get Obliterate until 58, going from Death Strike to Obliterate is a lot more intuitive than going from Death Strike to Festering Strike, or switching to Festering Strike from a weird combination of Icy Touch, Blood Strike, and Scourge Strike.
That said, there's still other options that should go lower too. Improved Blood Presence, for example, deserves to be lowered to level 55. As someone whose been tanking since Vanilla WoW, I can tell you being critically hit can seriously destroy your tanking potential. Death knights who want to level strictly as tanks should have that option, and while being critically hit in a lower level dungeon isn't the end of the world, why arbitrarily deny a tank that basic cornerstone of tanking for so long?
A refreshing look at diseases
It's an open secret that pretty much nobody uses Icy Touch and Plague Strike for disease application these days, at least not to the same level we used it in Wrath. If you want to apply diseases, you're using Outbreak (or the Ebon Plaguebringer version of Plague Strike for unholy), then keeping things going with Pestilence, Festering Strike, Scarlet Fever, or Unholy Blight. This is one reason why it's a little silly that we still have to wait until level 81 for Outbreak, or even level 68 for Ebon Plaguebringer and Scarlet Plague, especially since Unholy Blight is available almost immediately. Disease Control is so much more than simply swapping in Plague Strike and Icy Touch, let death knights just have their hands on it all as early as possible. If you want death knights to learn disease management before they get all their tools, it would be better to give them Outbreak right away and just save the Unholy Blight/disease control tier for level 75 (thus making it another candidate to switch with the level 75 tier as above).
There's other arguments to be made, but for the most part, these are the 3 biggest issues I see, since they all prevent the death knight from having a solid grasp on the very basics of their class. Other stuff is fine to be granted later, such as various proc abilities, or extra passives or survivability buttons, but you need to have the basics right out of the gate to allow a player to be familiar with the basics of their class as soon as possible. With these changes, new death knights will have that again, and will have a chance to level with confidence and ease to become the beautiful raiders, PvPers, and daily machines we know they can be.
Learn the ropes of endgame play with WoW Insider's DK 101 guide. Make yourself invaluable to your raid group with Mind Freeze and other interrupts, gear up with pre-heroic DPS gear or pre-heroic tank gear, and plot your path to tier 11/valor point DPS gear.