Be the hero of heroic scenarios
One of the new features I'm most excited for is the addition of heroic scenarios. These scenarios will give you a chance to face a harder version of current scenarios for a significant chunk of valor points and a chance at some raid finder level epic gear. While these scenarios, in theory, are still balanced so you don't need a tank or healer, a quick run of a couple on the PTR will reveal that it definitely helps.
A good example is the Dark Heart of Pandaria scenario. The final boss summons adds, sometimes emits clouds of smoke that do heavy damage to anyone caught in them, and casts some nasty spells that should probably be interrupted. Being able to wrangle those adds in, interrupt most of his casts, take his hits, and be able to keep his attention long enough to pull him out of the smoke clouds is something a blood tank is definitely suited for. Consider it insurance to allow yourself to finish the instance and get your valor points.
Alas, the downside here is that heroic scenarios require a relatively high item level, which means your tanking off-set might not cut it. Luckily, that's when another little adjustment coming in patch 5.3 does right by the death knight.
The loot specialist
It's a common problem I've seen this expansion. You're a dedicated DPS death knight, but you like keeping up a tank set so you can be ready for whatever comes. The problem is, your tanking set is mostly scattered heroic dungeon drops, because you need to go to Raid Finder as DPS because of how crappy your tanking set is. Some people have gotten around this by DPSing as blood specialization, but that's not something I would recommend, since it essentially makes the rest of the group carry you.
Patch 5.3 is fixing that by allowing you to choose which loot specialization you're rolling for, both on Raid Finder boss drops and on any bonus dice rolls. This way, once you're happy with your DPS gear, or even if you're just slumming in Raid Finder, you can switch your loot to tank spec and get some extra tank gear. If you're a frost death knight who's really desperate for a two-handed weapon upgrade, you can even take one-handed weapons out of the equation by switching to blood loot.
Get new loot
Since there's no new raids or dungeons in patch 5.3, there's not a huge amount of new loot in this patch. Still, you do have one option. Latent Kor'kron Armor can be purchased from the Darkspear Rebellion quartermaster or found more rarely on mobs in the Northern Barrens. When combined with Radical Mojo, you'll get a piece of item level 489 gear with a random enchantment based your current specialization on it. The gear is generally styled after the old dungeon 2 tier gear from vanilla WoW. Since death knights didn't have one of those sets, we instead get a recolor of rare starting zone gear, Saronite War Battleplate. This stuff isn't going to be an upgrade for you if you've been gathering Throne of Thunder drops, but it'll be useful for more casual death knights, non-raiding death knights, or for off-specs or shoring up weak spots.
Runeforging and the future
One thing we won't be getting in patch 5.3 is new runeforges. That said, Ghostcrawler recently replied to a tweet noting that they are not happy with the runeforging system and hope to revamp it in the long term. Personally, I wouldn't expect to see any changes until at least patch 6.0 (and possibly not even then), but it does bring up some legitimate questions about the system.
One camp, which I have to admit I am often a fan of, is the "if it ain't broke, don't fix it" camp. We have a solid set of options. DPS can get a boost of strength and a bit of healing (and a bit of extra frost damage if they dual wield), while tanks can choose from a solid bit of parry or some extra health and armor. It doesn't hurt that being able to runeforge saves quite a bit of money on weapon enchants over the course of an expansion either. Given that our runeforging options are consistently better than any enchanter's enchantments, It's difficult for me to complain about it.
What reasons might there be to change it? There's something to be said for variety, and death knights have been using the exact same weapon enchantments since their inception thanks to runeforging. Of course, you could alternately argue that most weapon enchantments don't vary too much anyway. In many cases, all that changes is exactly which stat they proc. You could also argue that dual wielding has an unfair advantage over two-handed thanks to the ability to use an extra runeforge. Are either of these reasons to completely overhaul the system? I'm not sure I see it, myself.
Regardless, this is a long term project of the dev team, and it's likely that whatever form it takes, it will be different enough for the current system that it might not be easy to speculate. We'll just have to wait and see what they come up with.
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.