Many casual players have discovered the wonders of making a Death Knight since WotLK was released, but in case there are still some holdouts, here are some reasons why those of us with limited playtime should definitely give the class a try.
Seriously. Widdle gnome Death Knights with pigtails destroying multiple mobs 10 times their size are worth the price of admission. The spousal unit and I went back to our neglected Alliance server just to make a gnome DK duo. Our guild there has long since dispersed and our friends are all on different servers, but who cares? It's all about fun and tiny agents of death = fun. Awww, dks in love.
6 more serious reasons for a casual player to make a Death Knight are after the break.
6. DKs make excellent alts.
Whether you use alts to maximize your rest (good idea), keep your WoW time fresh or escape from bad situations, it's good to have at least one playable alt around and Death Knights are an excellent choice. If you've been playing this game a while, you've probably done all the starting quests a few times and are tired of them. With a Death Knight, you hit the ground running at 55, the first three levels are done before you know it and you're ready to zip through the Burning Crusade.
5. DKs make excellent gatherers.
Though they hinted a couple BlizzCons ago that Death Knights wouldn't have to go back to Elwynn Forest to kill wolves or pick flowers in order to level their professions, nothing like that was ever implemented. So, it doesn't matter what professions you choose, you have to start right along with the level 5 characters. That having been said, the fact that you get an epic mount for free (well, you steal it) really helps catch up to your level -- at least in the gathering professions. The cost of leveling the crafting professions is rather prohibitive, but gathering is a quick and easy thing to do which adds useful abilities, feeds your other crafters and can net you lots of extra cash on the AH. I particularly recommend Herbalism and Skinning. You get a free heal and a crit buff, they are the quickest of the three to level and you don't have to switch your minimap find function.
4. Death Knights are EZ mode.
Maybe you haven't read about the best builds/gear/techniques for playing your Death Knight, but for leveling purposes, you don't need to read a thing. Just pick the talents that seem fun and start mowing through your opponents. They are a bit overpowered while leveling, allowing you to solo even elite mobs above your level and defend yourself against the DK hating gankers (of which there are a few on PvP servers). They are also really good at fighting multiple mobs at a time, making questing that much faster.
3. Warlocks in plate.
This is kind of a repeat of number 4, but soloability is important to those of us whose schedules don't allow for much grouping. If you go Unholy spec, you get a pet ghoul that obeys your commands and is ready to fight whenever you are. Well... a couple seconds after you are... he has to crawl out of the ground first. With very little effort, dying is a rare event and everything short of instances can be done by the two of you.
Having arms pop up out of the ground while you fight is delightfully creepy goodness. Yes, I am biased toward the Unholy spec.
And the number one reason to make a Death Knight, whether casual or hardcore or anywhere inbetween:
1. The starting quests.
The lore, the instanced world, the quest chains, Arthas whispering in your ear -- the starting DK quests are sheer gaming pleasure. I have never had such a hard time reconciling my personal ethics with what I was doing in-game (yes I did things the hard way -- the light side -- in KotOR) and I loved every minute of it. Even if you abandon your DK right after your walk of shame to Thrall or King Varian Wrynn, the first three levels of being a Death Knight is worth spending a few of your play sessions.