1) I am playing an Guardian and i have avarage ~730 DPS throught the fight. Is that enought?
730 DPS sounds low. If you are in full 72s, you should do more than that. As an example I'm a jugg tank doing NiM content and I usually average between 900-1200 dps during boss fights. This is in full tanking gear (ie endurance crystal, guardian hilt / armoring etc). How much DPS I put out relates to if there are boss switches / how long I get to stay on the boss, as rage becomes and issue when I'm not tanking the boss.
2) Should i exchange some of my deffstats to offensive stats like power or ap?
Definitively no. You are a tank, and you should gear as a tank. If DPS is the issue, then your damage dealers should take a look at their rotation and set up, not you. Btw, what is their DPS? A lot of the fights in NiM have DPS checks.
3) Or shoud i go for accuracy?
Again no. Taunts always hits, and so does your aggro-building attacks, ie. Smash / Chrushing Blow / Retaliation / Ravage / Backhand.