World PvP will cause balance problems as Mixing PvP and PvE will make it impossible to balance each side without effecting the other.
so IMHO world PvP will only bring harm for the PvE community and ruin the game for the majority.
on the other hand Arena PvP will offer a seperate place for PvP which allows them to balance skills without effecting the PvE version but it isn't just about balancing the game, Arena PvP make it easier to find matches with the auto match making system and it always puts you against someone at your skill level so it makes PvP always fun whatever your skill level.
still i think blizzard can do more with arenas like:
1-adding different game modes like (FFA, CTF, Horde, etc)
2-allow manual match making where you can choose who to play with and against (which makes 1v1 duels possible and playing uneven matches with sets like 1v4 or 2v3)
3-track stats for each person (for people who don't wanna show it, they can make hide stats as default)
we don't know what blizzard will do with arenas but hopefully they will add some of these ideas and they said they are open for suggestions.
bottom line, i find Arena PvP has more potential than world PvP and best of all it doesn't hurt the PvE community like World PvP does and even with the little info we know about Arena PvP i prefer it over World PvP.