"Clunky" is a descriptor that shows up in a lot of balance complaints. I think I've probably used it once or twice myself. Ghostcrawler recently outlined some of the issues with it on twitter. The main issue is, "Clunky" needs to mean something specific. It cannot simply mean "a mechanic I don't like." Among the dev team, says Ghostcrawler, it originally meant something that was unintuitive and required extraneous steps to achieve.
Thus, while we might say the unholy rotation is "Clunky," what we need to describe is why it's clunky. In the case of unholy, It may be best to focus on the differences between it and blood and frost. The very fact that unholy uses a different rune spending scheme could be seen as a huge learning curve for new players who want to switch to unholy, especially since both frost and unholy focus their fighting on a 2-rune strike, whereas unholy is constructed around a 1-rune strike. Is that in itself enough to be clunky? Perhaps, but then again, the fact is you are switching to a new spec. Within the spec, and only the spec, is that rotation still relatively smooth? Arguably, yes. Probably the biggest issue was with disease application, which Blizzard arguably fixed when they put both diseases on Plague Strike for unholy.
That said, if there's one place where we can arguably use "clunky," it's probably in regards to the unholy AoE rotation, which still requires you to go into the situation with death runes pre-loaded for the optimal setup, and can still easily result in orphaned frost runes as you try to use Blood Boil as much as possible.
Scaling and fishing for buffs
Scaling as a term, is one that is used even more than clunky, it seems. I may say this in part because I tend to be a pretty huge transgressor for this specific term, to be fair. In theory, scaling as a term refers to your abilities gaining in power as you gain better gear. To be honest, every ability that does damage scales to some level. There aren't very many abilities that don't get at least some help from attack power and/or at least one secondary stat.
So when we say something doesn't scale well, what exactly do we mean? For death knights, this usually comes attached to weaponry. As physical DPS, weapons are our alpha and omega. A better weapon will nearly always boost your DPS more than any other upgrade of similar power. This is not only because you do more base damage, but because, since so many of your abilities are attached to weapon damage, even your special abilities will get that much more powerful.
But this definition doesn't answer the base question. Is lack of scaling, whether true or not, a valid argument for class balance? Scaling may be one of the bigger arguments because of how much a new weapon changes everything. Getting a new weapon is so important to our class that we want to feel that much more powerful when one drops.
But in the end, the fact remains, everything scales to some point, and there's never really been a sacred writ anywhere that says an ability must scale at a certain rate or be tied to a certain mechanic. Scaling arguments generally seem to be a shorthand for frustration at balance issues. If you think your class' DPS is too low, saying that you "scale bad" can be a way to try to justify that and find a way to demand a fix. "Scale bad" seems to be shorthand for "let my abilities do more damage."
And perhaps, in the end, we should be honest and stop using the term "scaling." Whether an ability scales a certain way shouldn't matter. What should matter is if our damage is competitive and our abilities all work together relatively well.
Horrible amounts of class
This is not to say those who complain about class always use high-sounding terms like "scaling" or "clunky." Sometimes, it's a lot more passionate. Sometimes it's "my class is bad." The biggest issue here is that sometimes "my class is bad" may actually mean "I am bad at playing this class."
The easiest way to figure if this sentence is true is to look at the class' performance on the community stage. What type of DPS are other people of your spec and class doing in raids? What's the average DPS expected of you? Are you meeting it? If so, can we parse your logs and see if you're actually following the standard DPS procedure expected of your class? On the tank side, are you using the right cooldowns to survive? Are your healers up to snuff? This is probably the easiest of the complaints to disprove, if the complainer freely gives up his or her log data.
This is not to say that any of the above complaints are always invalid. There's been times when the death knight class, or at least specific specs, have in fact arguably had "clunky" mechanics or had a spec that was just plain terrible. Luckily, we have smart and dedicated theorycrafting community members (such as the Archerus group) who can analyze the issues and get, for example, the type of unholy buffs we got a few patches back.
When the Warlords of Draenor beta drops, whether that's in a week or in 6 months, we'll probably hear all these terms being used in class balance arguments, and maybe even use a few ourselves. In the end, though, if we want to affect real change for death knights, it's always good to know why we're using the terms, and to have the data and reasoning to back those terms up.
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.