What MMO's in general need most to be fun is challenge and community.
This does not necessary mean PvP, questing, high levels or big loot. But rather a mix of that.
PvP is the easiest way to provide challenge to a community, because members will have to challenge eachother instead of the word around them. If PvP is implimented in a bad way it backfires however and 'challenge' soon becomes 'annoynce'.
PvE encounters need to be designed and tested by Devs, and then done by players. And then players need to have fun doing them. What makes a good encounter is on that isn't to hard (this is the 100th time we try this, now it WILL work!) or to easy (lets go kill this and that). However, if the hard encounter has a reward that's worth the trouble (Aen armour for example) people will be challenged to do it, even if they succeed only once.
The easy encounters will become straight boring, and used for 'farming' if there is nice loot (high level players killed lower level bosses).
One of the things that makes the fun is randomness, which give the "I didn't expect that, awesome!" kind of feeling. This happens more often in PvP then in PvE, as PvP is normally 100% player action, and players don't use predefined actions and scripts to decide what they will do.
In PvE the randomness has to come from smart scripts, in other words, Dev work. The predator/herbivore behaviour sometimes gives this, as does the spawning. It's avoidable (as any forager will tell you) but helps making the game fun.
The basic conclusion for me is that I don't find PvP needed, but it is the easiest way to implement challenge as you only have to provide something to fight over (read Outposts, spires and whatever else is comming). You have to do it the right way though, else it won't be easy, only even more work.
-- EDIT, thanks Irving. Unfortunatly that Latin thing won't work for me. I don't know any of it
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