Ramping up and bursting out
Ramp up time essentially refers to the time needed to get to your full burst damage potential. The longer this time, the longer the battle needs to be before your true potential is unleashed. All death knights have at least a little ramp up time in that they need to apply diseases to power up their strikes, but unholy death knights take it to a new level, primarily due to the problem of their ghoul. The ghoul does a good chunk of the unholy death knight's damage at all times, but when you add in the Dark Transformation buff, things really get good.
The problem is, to get Dark Transformation up, you also need 5 stacks of Shadow Infusion (which in itself is a needed damage buff), which can only be applied by Death Coil. This means that for a good portion of a battles beginning, a death knight is struggling to reach full power, and gains and loses power on a consistent cycle. This can be especially fatal in PvP battles where burst damage is king, but also means boss battle DPS becomes a bit more capricious and harder to control. Finally, disease damage is a much larger part of the unholy death knight's arsenal (though not as large as it used be), and thus it's more important they keep diseases up at all times.
Frost overcomes this by essentially needing only diseases up to reach full potential, and even then, they have little trouble keeping up Frost Fever thanks to Howling Blast. In addition, their spec-specific burst damage button, Pillar of Frost, can be pushed on demand.
The most popular fix requested for this issue has been to reduce Shadow Infusion to something like 3 stacks, making the time needed to apply it to full power and hit Dark Transformation that quicker. There's also some suggestions Dark Transformation should be able to be hit at any time and ghouls should automatically do more damage rather than relying on Shadow Infusion stacks.
With lots of the latest tier 14 raid fights supporting AoE or "Cleave" damage, any class that can do better AoE has an edge. Frost death knights can definitely be some good AoE thanks to Howling Blast. Unholy death knights are, alas, a little less accomplished at this. Essentially, they spread diseases, lay down Death and Decay, and try to unleash as many Blood Boils as possible. In the end, it just doesn't keep up with Howling Blast, and that means they fall behind.
In part, Blizzard has tried to fix this by making Reaping apply to Blood Boil, but that still means you need Festering Strike to convert the frost runes initially. One popular fix here is a request to make an unholy-specific version of Blood Boil that costs one frost and one blood rune, thus both fixing the awkwardness of the unholy AoE rotation and allowing for adjustments for a harder hitting AoE specifically for unholy to compete with Howling Blast. While there is an argument here that such a move would be strange and confusing, I have to say, frankly, that it would be no more confusing that the pure existence of Festering Strike and the one-rune Scourge Strike we have right now.
Spending runes the unholy way
The advent of the one rune Scourge Strike came at the same time that Blizzard revamped death knight trees and ended the Wrath experiment of allowing all trees to tank and DPS. There were many reasons given or implied for this switch. One was to give Scourge Strike a unique feel apart from Obliterate. Another was to provide the "one rune" DPS style of blood's Heart Strike to another tree, now that blood was going pure tank. Regardless of the reasoning, though, unholy has stumbled through a full expansion and into a new one with this setup, and while it's not the sole reason, it has served to make the tree a whole lot less appealing.
To start with, it's confusing for new players to get into. Since they don't even start on a new death knight with Festering Strike ready to, they end up either using scourge strike while leaving runes hanging in the wind, or using a basic Death Strike/Blood Strike rotation. Then, they hit level 64 and only then have all the basic tools they need to accomplish their rotation. Even then, they'll be missing Outbreak for quite some time, which is arguably needed to effectively apply diseases without orphaning runes.
So this new setup makes it harder to use survivability measures like Death Strike, makes it harder to apply diseases, and makes it more awkward to switch between specs and learn a whole new button setup. Frankly, it's long past time that Scourge Strike went back to a frost/unholy rune cost, perhaps gaining Festering Strike's disease refreshing power in the process, with blood runes being saved for Blood Strike. It will make the spec a lot less confusing, it will make it fit in smoother with the class overall, and it may attract back a lot of players who have switched back to frost out of despair and confusion
Can we fix it?
In the end, of course, much of this is stuff we've been saying to Blizzard since Cataclysm. It may be that Blizzard is happy with the style of unholy as it currently rests, and unholy will have to remain a tree for those who hate frost enough, or like unholy enough, to put up with its quirks and difficulties. At the least, sims suggest you should be able to remain competitive with frost if you work at it, you'll just have to get really good at unholy DPS.
That said, there's a lot to be said for continuing to speak our mind in hopes that we recover the glory days of unholy. We may never be as overpowered as we were in the early Wrath era, but it'd be nice to have that play style back, at least. Prominent Acherus community member Magdalena has started a thread on the official forums hoping to collect unholy death knight raid parses as a starting point. Blizzard has shown a willingness to listen to theory crafters bringing hard data, so it's a great place to start.
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.