After release, they were known as op, however the got neffed. Theres no best class as all classes have their own features and specialities.
Absolutely not. There is not a "worst class ever" class. Each class has its own strengths and weaknesses.
Look on this website in the related links below, then go under the class for Death Knights. Im sure you will find an addon that would be nice for a death knight.
Death Knights of Krynn happened in 1991.
Death Knights of Krynn was created in 1991.
You can have as many death knights as you want on your account, but you can only have 1 death knight per server (example: you have 5 death knights - 1 is on agamaggan, 1 is on doomhammer, etc.)
no
Worgen and Goblin will be Death Knights, because it says it on the site of Blizzard. They just might not be available right away when Cataclysm is released. This is due to Death Knights having a head start in the leveling process.
In the related links section below are two builds, one for tanking and one for dps'ing.
Yes, they can.
Theoretically, kings paid knights for their services as mounted warriors. Most knights came from the landowning class, so their wealth was their land.
World of Warcraft: Yes, Death Knights can wear the Bind to Account heirloom armor if they have an 80 on their account with the means to get it.
Death Knights can be a tank and dps. It all depends on where they spend their talent points and the gear they have. They may not be the best main tank, but they are defiantly great off tanks. In the upcoming expansion Cataclysm, due out in late 2010, the Blood tree will be the tanking tree.