No, the Death Knight did not ruin WoW. Anyone who says they did looks at theory and not practice. Let me give you an example of what I mean:
"Well its overpowered coz its a hero class which makes it suck. It has a pet like hunter, It can gain health back, its just stupid"
Let's extract hard viewpoint from this. According to this line:
:- Death Knights are overpowered because it's a Hero Class
:- Because it has a pet, it's great
:- Because it can regenerate health, it's great
Now, anyone who's worth listening to is going to take those facts, be told they're apparent proof of how godly Death Knights are, and they're going to laugh at how flimsy and ignorant thse arguments are.
Death Knights are fine. They're equal tanks, they're equal DPS. No idea how they are in PvP, but from my perspective, which is raiding and heroic instances, they're just as good as anyone else, which lends credence to the goal of "Bring the player, not the class".
If you ask me, WoW has gotten better and better as time has gone on. Anyone who says otherwise is either a very in-depth player who enjoyed a certain way of playing and refused to adapt to suit new changes, or an ignoramus who needs to stop being ridiculously hypersensitive to change. Here's a good example of that last part:
"Let's be honest WoW was dead as soon as PvP was changed, 40-man raids were abolished, the level cap was raised and futuristic space-land was added. Which all happend at the same time. It's called the Burning Crusade and it RUINED LIFE."
Literally, the poster said "Four major places have changed" - but all of them changed for the better. PvP was made into an Arena system, that demanded actual ability instead of grindfests (as an aside: I ground my way to General ranking in the old honour system and I managed to rack up a 1900 rating in my brief Arena spat. The latter was FAR more preferable because it demanded that you were GOOD at what you do instead of having TIME to do it alot. The former was simply a timesink that demanded you run with a large premade against PuG teams repeatedly, ad nauseum), 40-man raids were abysmal compared to 25 and 10-man variants, the level cap being raised effectively does nothing but add more content to enjoy and a new setting is a personal matter, but I didn't really mind it.
Sadly, you get many people who will simply say "The expansions killed WoW" regardless of WoW's major growth, total enjoyability and depth. These people were simply unwilling to change with the game, and therefore, while others bent like bamboo, they broke like dry twigs, and their false wailings can be heard throughout the internet.
Phew. That's some wall o' text.