In short, no matter what mechanical issues you may have with a death knight, it's still a good time to play one. Most complaints that are left are things that are quirks of the class we should probably learn to live with. Of course, I wouldn't say "no" to recent attempts to make raids more melee-friendly, but that's more of a role issue than a class issue.
Why is haste still good for me?
The consensus this tier for frost death knights is that haste is out. It's just not very good anymore. Instead, your focus should be on critical strike and mastery. This has confused some two-handed frost death knights, who find that haste works just fine for them when they run sims. There's usually one major culprit for this: Plague Leech, or the lack thereof.
Plague Leech has always been the "best" first tier talent for death knights for DPS. Simply use it right as your diseases would have expired anyway, and you've got an extra death rune. However, up until patch 5.4, many frost death knights took Unholy Blight anyway, because Plague Leech was considered too much of a hassle for too little return.
This changed in patch 5.4. Blizzard buffed Plague Leech so that it now reactivates 2 runes as death runes. While the annoyance factor is arguably still there, 2 runes makes the boost too considerable to ignore.
If you take Plague Leech and use it correctly and frequently, haste is no longer such a factor. If you're sticking with Unholy Blight, there's a chance haste will still be boosting your DPS more. You need to make the choice: Do you want the convenience of Unholy Blight, or do you want to start learning how to use Plague Leech for a not-insignificant DPS boost? Depending on your answer, your gearing choices may change.
Fixing low DPS
This complaint reaches beyond 5.4 and will probably always be with us, but I've seen it enough lately that a quick refresher couldn't hurt. There are many factors that could making your DPS too low, both gearing and play style related. The reasons someone may be struggling with DPS may not be the same reason another person is. We'll quickly go through some of the most common reasons.
Using your DPS cooldowns is one of the most common reasons you could be seeing low DPS. With a trinket this tier that lowers DPS cooldowns this tier, this reason is even more pertinent than ever. Of course, there's your obvious cooldowns, such as Summon Gargoyle and Pillar of Frost, but one that a lot of death knights overlook: Anti-Magic Shell. While it may seem primarily like a defensive glyph for absorbing magic, it ends up being a major DPS boost when you use it right because it gives runic power. Use glyphs to get the most out of it, then use it to soak damage as often as possible, then turn that damage into a few extra Death Coils or Frost Strikes.
On the gearing side, you want to make sure you have hit and expertise to 7.5% each. This will make sure that all your special attacks hit any raid boss. They will still be able to parry, but you can fix this by standing behind the boss when attacking, since they can't parry from the back. Get to these 7.5% numbers however you can, be it by reforging or by putting rigid gems in your blue gem slots. That said, the stats are mostly useless after 7.5%, so you also want to make sure you don't get too much hit or expertise past those numbers. Still, ultimately, it is always better to hit with your attacks than miss, and ounce for ounce, getting hit and expertise to that 7.5% number will almost always be a better way to increase DPS than any gear related thing you can do.
Also on the gearing side are the practices of gemming, enchanting, and reforging. When putting gems in your gear, simming to find the best option is optimal, but a general rule of thumb is that you go for your best secondary stat and go for status bonuses. So, for example, if you have a red gem slot, and your best secondary stat is critical strike, you'd probably want an Inscribed Vermilion Onyx. This would allow you to get some critical strike chance while still fulfilling the requirements for the slot bonus.
Reforging is generally the best place to get some extra hit or expertise if needed, or shedding some extra hit or expertise if it isn't. Otherwise, you should focus on getting rid of your weakest secondary stat in favor of your strongest secondary stat. For example, if you pick up a piece of gear with haste and mastery, chances are you'll want to reforge the haste to critical strike. If you pick up a piece with critical strike and haste, you will probably want to reforge the haste to mastery. If you aren't sure, Sims are best, followed by a tool like Ask Mr. Robot.
Enchantments should also be taken care of. These are usually pretty easy to grab just because there's only one or two viable choices per slot. Don't forget to get an inscription for your shoulders, a belt buckle for your belt, and leg armor as well.
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.