what are the criteria for becoming a Death Eater??? and is harry really a half-blood?

[kashelkar]

No: HPFGUIDX 92840

Hi,

It really seems weird that during the fight between the DA members 
and death eaters towards the end of OOP, the DEs don't demonstrate 
any kind of special (Dark) ability. Is just knowledge of unforgivable 
curses (apart from worshipping Voldy) enough criteria for becoming 
DE?? Bella tells Harry "....I've learned the dark arts from Dark Lord 
himself..." What are these dark arts? How come she doesn't use them 
while in combat with the DA members?? How come the knowledge levels 
of the most feared DEs are just the same as the 5th year students at Hogwarts (which doesn't preach dark arts.)???

On a different subject

Harry was born to a witch and a wizard (both from the magical community), so isn't he a pure-blood? I think to be a half-blood (like Voldy), one of the parents has to be non-magic person.

~kashelkar