NiKr, in your experience, if this was counted not in Open World but primarily for Guild Wars, would that work?
I meaaan, isn't Guild Wars related rewards already the rewards for killing people during said wars?
Though depending on how they set up the war dec system, it might still not work. Even if it's purely achievements and nothing else, there'll be people who really want those achievements (namely me). They'll make the smallest possible guild that can be decced with their and their friends' alts and just farm that shit in no time. Steven already said that you can kill war enemies however much you want, so yeah.
I could maaaybe see this kinda working with Node wars, purely because the cost of that kinda thing would be much greater, but then you could just pay someone on the other side to die to you a few hundred times. Or hell, you could just have a friend in that node and get the top achievement for each other.
I'm not saying this isn't a cool thing, and I myself have pvped very hardcore in L2 because it had top pvp leaderboards on the server's site, but quite often those places would have at least a few people who cheesed it. And while in L2's case those people would be dunked on and shunned, because they'd usually be shit in proper pvp, with an achievement system your rep wouldn't matter as much. It's not like someone a year from game's release would know for a fact whether you cheesed it or not. And imo that kind of interaction dilutes the value of the system, which is why I'm against it.