For players who have experienced much of the current endgame, leveling a Death Knight won't be as limited an experience. But for some players whose current characters only meet the minimum Level 55 or not much higher, a lot will be missed. This is certainly true for all players who started playing the game late, but glaringly so for Death Knights, who don't even figure in the loot tables. Will Blizzard bother to update Tiers 4 through 6 for Death Knight armor? I'm inclined to think no. Very few Death Knights will experience Karazhan. They won't need to. By the time Death Knights are high enough to enter Karazhan, in the weeks it takes for the instance to reset, they will have leveled past the place.
Playing a Death Knight means playing catch up in every sense of the word. The game wasn't built with Death Knights in mind. They're being retroactively fitted in, and because of their Heroic status simply don't have anything to do with the game world below Level 55 (technically 57-58, which is the level most Death Knights will enter the rest of the world). As much as Blizzard tries otherwise, the true balance of classes begins at Level 80. This means that Death Knights have to hit max level in order to finally be on truly equal footing.
One of the problems I've seen with Beta is that many Death Knights have found difficulty finding groups for instances in Northrend. Because of limited play time, Death Knights at Level 70 generally aren't as geared as other classes who've been around since before Wrath. Most of the time, those running around Northrend will be equipped in quest greens and a few blues; and while it really shouldn't be a deterrent for groups, there are those who won't take a poorly equipped character into an instance. Much less a class that few people know how to play.
Let's face it, starting Heroically doesn't necessarily equip a player with the knowledge to play a class. Death Knights will have to catch up on class knowledge intended to be learned over the course of leveling 55 levels. There are no precedents -- Beta or PTR information notwithstanding -- and all Death Knights will have to catch up on about four years of knowledge amassed for other classes. When we group with a Priest or Warrior, for example, we know what to expect. We know if those players are playing well or know their class. When we group with a Death Knight, we don't really know what to expect. If they perform badly, we're not quite sure if it's because the player sucks or because the class still needs a little work.
If a Death Knight's DPS is sub-par, just how well do we know Runes and Runic Energy to determine if they're using the right rotation? Is it gear? Is it the way they play? Or does the class really does have some kinks? Without the benefit of experience like we have with all the other classes, gauging Death Knight performance can be difficult. Of course, right now in Beta the class feels overwhelmingly powerful. The developers are sure to put this power level more in line with other classes when the game finally comes out, though. When that happens, players must work extra hard to prove themselves.
For certain there are those who will excel in playing the class. There are players who will quickly get the rhythm of Runes and unleashing Runic Power (it's not always for Death Coil, you know). But the general Death Knight populace will need to catch up with the rest of the community. It isn't even like Horde Paladins or Alliance Shamans. Those classes had been around for a while and players knew what to expect, what gear to get, and generally fit in quite easily into groups thanks to a healing and / or tanking tree.
For the very first batch of Death Knights, finding groups of the same level will be a problem as most of the populace evacuates Outland. The most likely path of progression would be to solo most everything -- missing out on a lot of content -- until you reach Northrend. Fortunately, midway through the new continent, gear parity will begin to set in. Only those in Tier 6 or Sunwell gear will be able to keep their gear intact all the way through Level 80, sometimes not even. A lot of good replacements will be available such that as you reach max level, gear discrepancy will be less of an issue.
The great thing about Northrend is that it's an equalizer. A large portion of the player base will go through gear upgrades while making their way to Level 80. The only real problem is the chasm between making your Death Knight and getting to Level 68 (the supposed minimum for Northrend). Unlike other classes starting from Level 1, where it's easy to find similar-level characters and form a group, the first batch of Death Knights are likely to be grouped with... other Death Knights. These are minor set-backs, of course, and merely part of the challenge in rolling the class.
The pressing issue is amassing the knowledge base that other classes have gathered over four years of gameplay. For certain, a lot of number-crunching will be done on the many sites devoted to such things and optimal rotations will be uncovered. But the great thing about millions of players is discovering how to play the class in a way the developers never expected (or intended). The best specs and talent synergies are revealed through constant gameplay, and this includes Level 80 Arenas, the big unknown as far as Death Knight performance is concerned. If you plan to create a Death Knight and level it through to max level, it becomes your duty as a player and pioneer to close that knowledge gap.