Edit: Follow up post.
Blizzard games have always traditionally been about the "Easy to play, hard to master" mentality. Unfortunately, I don't think the DK quite falls into the "easy to play" part just yet.
It is a rather complicated class, and the mechanics can get a little clunky sometimes when things don't quite fall into place. It can be a lot of work for a new DK to not only learn how to manage multiple resources instead of one like they're probably used to, but to also be required to do their moves in specific orders, so as to get the most bang for their GCD spent.
Is that a good thing? For the sake of keeping things interesting, I suppose. But from a competitive standpoint, I'm not so sure. I'll give examples of why down below.
If you were to ask me (and I'm probably going to catch a ton of flak for saying this), I would suspect that the single biggest culprit of the challenge of playing the class properly, is the over-dependence of having diseases up on the target to do maximum damage with most of your strikes. It's a cool mechanic, until you start running into situations where rotations simply aren't feasable (I'm looking mostly at pvp here).
I have this suspicion that, if anything, this is the department that will probably need to be looked at going into the future, if things prove too difficult or DK DPS is too inconsistent depending on the opponents they face.
One example of a class that was probably too hard to play well in TBC (if you chose to forgo the way of mods, macros and a G15 keyboard), was the hunter. Not only were they arguably the most difficult class to play well in the arena for a myriad of reasons, but they were also the most difficult class to properly DPS with in raids. One tiny, simply fix to alleviate their huge learning curve, was to remove auto shot clipping. Bam, much easier to play, and now will also be easier to dish out the damage in pvp without worrying about clipping shots while you're constantly repositioning yourself.
Also, try to sit back for a second and imagine for a second if rogue DPS strikes like Sinister strike and Hemo (or even eviscerate), were balanced around the assumption that to do max damage, you HAD to have some sort of deadly or other poison up at all times, otherwise you'd take a pretty huge DPS hit. On top of keeping up SnD, managing combo points, possible positional requirements, cooldowns, and keeping enough energy in reserves to kick/etc, you'd also cringe everytime your poisons got cleansed.
Not only would rogues be frustrated that their "MS" and snare poisons would fall off, but their hemo/SS damage would be less as well.
Fortunately for rogues, this isn't the case. They can do all their high damage moves without worrying about this, with the exception of mutliate. However, as in mutliate's case, it isn't so bad, since Assassination specs have typically have their target poisoned the vast majority of the time, if not by auto attacks alone, but also easy access to shiv which is essentially on demand, and doesn't run them the risk of screwing up any "rotations".
I guess what I'm trying to say is, is that I probably wouldn't rule out the possibility that DKs might end up needing some sort of disease dependency rework in the future. Would it dumb things down? Yeah, but if Blizzard has learned anything from TBC PvP, it should have been that overly complex classes that take more effort to play well fight an uphill battle against classes that are at least as effective with far less effort.
That, and I guess I'm trying to say that our strikes and abilities should provide strong incentive to use on their own, and not require us to use X, Y abilities before we can even think about using ability Z. If anything comes along to screw up that pattern, then everything gets a little out of whack, and the whole process just feels very much out of place. Whenever that happens, I always sit there and think to myself, "Well Gee, do I just start my whole rotation over because A) My disease got cleansed, or B) I had to use chains of ice instead of apply frost fever via icy touch? Or do I just go ahead and blow a 1F1U ability that does subpar damage?" It's a double whammy. Not only is our utility/damaging disease removed, but our subsequent strike damage that would have followed suite also do less damage unless those diseases are reapplied.
It's... frustrating, to say the least. While learning how to pvp with this class, I'd often run into situations like the above stated, and just try and figure out in my head if it's worth it to just spam disease applicators and get as many quick strikes in? Or just forget about keeping diseases up and try to get those hard hitters off anyways, even if they do less damage.
After a while, I started to go with the second option, but even then, that doesn't work with an ability like howling blast, as it's damage is absolutely laughable without frost fever up. It struck me odd that what was once an excellent burst ability has now become a real pain to get working right in a competitive PvP environment.
I don't want to sit here and go into the frustrations I have with each individual strike or ability that suffers from this problem, as you can probably see where I'm going with this anyway. I just wanted to bring up the overall point that I'm trying to make, which is that the heavy disease dependencies are more frustrating than fun in pvp, and that our "rotations" don't apply themselves well in pvp, once we start needing to apply chains of ice as often as is required.