The Death knight is defined as the terminator of witchcraft characters, after weakened, the advantages for DK to witch disappeared; But this didn’t influence the strong position of DK in PVP. Look at the following review of experience DK and we can Achieve an overall feel of DK in WOTLK Power leveling.
There are many reasons for me to play Death Knight, and the reason related to PVP is : I believe the DK will be very strong in the future, and can adapt the environment quickly, at the same time ,in our server there are few players who are excellent to play this role in Wrath Of The Lich King Power leveling, so to get familiar with DK and master this role is an interesting challenge for me. 
Defend skills: Although Blizzard Entertainment define DK as the terminator of witchcraft characters. But in my opinion, from Strangulate in Beta version, and the weakening of shield to fight against magic, the DK is stronger in a close fight.
The winner is who are more stronger, and whether the enemies can realize the shortages of DK, Once the distance between witch and DK, the DK has only a way to shorten the distance between them, and the skill has only 25 seconds of CD after adding a talent in Wrath Of The Lich Kingleveling.
In PVP in WOTLK leveling, the DK has many skills which confuse the players, players should know their feathers and functions, learning how to deal with them and finally conquer them.