The first tier of our talent system deals primarily with the manipulation of diseases. This is, all in all, a relatively balanced tier. There's two small problems here. The first, and more minor, is that Roiling Blood doesn't fit comfortably into frost anywhere. This is one that probably won't need to be fixed, per se. Talents that are naturally better for specific specs seem like a reasonable thing, and Roiling Blood is very good at what it does.
What probably will need fixing is Plague Leech. Plague Leech is a talent that a lot of death knights have had some justifiable distaste for since it was first introduced. Because it supplies an extra death rune, it ends up being a straight up damage increase, but only if you use it just as your diseases about to expire. Interestingly enough, while it is a damage increase, most death knights seem to agree that it's not the end of the world if you don't take it. Part of this may be because it's only a damage increase if you use it exactly right.
Still, if any talent of ours is liable to be changed, I'd expect it to be Plague Leech. It does technically have an unfair advantage of increasing damage over the other two, but is similarly so complicated to use right that many death knights skip it anyway. Swapping it for a less awkward talent would be welcome.
The second tier is a set of survivability options. This one is actually pretty solid. Purgatory here is probably mostly blood-only since it's so hard for a DPS to get the healing needed to survive, but both Lichborne and Anti-Magic Zone have a lot to recommend to any spec depending on the situation. If there's an argument to be made here, it might be that Anti-Magic Zone should be baseline so that all death knights have a solid raid-wide cool down, but I wouldn't expect this tier to be touched.
If the 3rd tier has a problem, it's that Death's Advance is just too good. Having extra movement speed is very hard to pass up. One of a melee's main duties, whether tank or DPS, is getting out of the fire and getting into position, and a talent that helps you do that is pretty solid. That said, the other two options have good situational use. Chilblains is amazing for kiting duty. So there's possibly an argument for making Death's Advance baseline (or taking it out altogether) and placing another mobility or control option here -- maybe even a long-term crowd control option. That's probably something that would be more a decision to made in a patch 6.0, though, so I wouldn't expect this tier to change.
Tier 4 is our health regeneration tier, and has already seen quite a few tweaks. At this point, both Death Pact and Death Siphon are solid talents and unlikely to change. One offers a big chunk of emergency healing every few minutes, while the other allows on-demand healing as long as you're willing to give up a more hard hitting strike you could have use that death rune for. Essentially, you decide whether you want a strong burst of healing every few minutes, or quicker, smaller bursts at the cost of a chunk of your DPS. Conversion, on the other hand, is the black sheep of this tier, and for good reason. Since it drains a steady stream of your runic power, it essentially blocks the use of our rune regeneration talents and the benefit of Scent of Blood, resulting in a crippling us in ways the healing doesn't make up for. Conversion is right up there with Plague Leech for things that should be redesigned or removed.
The 5th tier, at this point, is what it is. At this point, it's an essential part of our class, and if it's ever removed, it's going to need to be part of a more complete redesign that you'd see in an expansion beginning patch. I would not expect to see any changes in patch 5.4. That said, what might be interesting here is a return of Blood Tap to a base emergency cool down as it was before the revamp, though it's unclear what one could replace it with.
I admit that I was not a fan of the final tier when it first came out, and I'd still kind of like damage talents as our top tier. That said, the tier as it's shaken out in actual game play has been relatively strong. Both Gorefiend's Grasp and Remorseless Winter have been very solid crowd control and mob wrangling options for trash pulls, and really have no pressing need to be changed. That leaves us with Desecrated Ground. While it has some limited use for PvP and specific PvE fights, it just doesn't have enough to recommend it in the long run. This is one that may need a revamp of sorts. Giving it a damage absorption component like Anti-Magic Zone might beef it up enough to be useful, though that might also beef it up so much as to make it mandatory, which wouldn't be any better. Arguably, it would also see a resurgence in use if one of the bosses in the Siege of Ogrimmar uses fear spells liberally, though that would still bring up a possible needed redesign for future boss tiers.
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.