Also, since the starting quests deck them out in blues, many feel godlike when they first step foot into classic/bc areas. In two senses, they are, as PvE solo mobs weren't made to be challenging based on such high level gear, and PvP fights against one other person is ridiculous when the death knight's gear is leaps and bounds above what other classes get until the end levels of Outland/early levels of Northrend.

However, this also created a huge flaw in the class, which is that they had only 3 levels with which to learn their class, instead of 58. This, paired with the easy-mode in solo situations, led to many* fresh DKs being the worst teammates possible for late classic and early BC dungeons, as well as battlegrounds. Quite a few times, when levelling my tankadin, Death Knight dpsers would just pull everything over and over, and the healer would quickly go OOM trying to save them, because if they didn't save them all that trash would wipe the group. Other times, their dps simply was too great for similarly levelled groups (around 55-60 range), and pulling aggro was a given for a person still learning the ropes of their class. On the PvP side, the early Death Knights made bgs a death-grip tug-of-war (I don't care what you may think, Death Knights, being death gripped 5 times in a row across the BG only to be put in Icy Chains at the end is NOT fun), and really could walk around with little to fear. Once people learned their weaknesses (and the got nerfed), however, it seemed this mindset persisted, and if you ever rolled into a BG with a 58 Death Knight, who just got [An Honorable Kill], you could rest assured they would be a liability for several matches until they learned what the heck they were doing.

Now, if I am levelling an alt and see a death knight before /I/ am 70, it is generally something to be avoided, whether it be my faction or the opposing one. I know there are good players who "get it", or have death knights on other servers and know what they are doing, but they seem so few and far between for that 15 level span before Northrend.