There is no PvP in Hellgate: London. None whatsoever. What's planned are competitive gameplay modes when you're not busy cooperating with others to slay hordes glass cannons. Example:
Two teams defending forts. A horde of monsters attacks each group. Killing monsters causes monsters to spawn faster for the other group. The overwhelmed group loses.
As for PVE, It's not like Guild Wars. Guild Wars PVE had monsters that were the same strength and hitpoints as players. It wasn't fun, because combat became tedious due to the abundancy of enemies and the amount of time and effort that it took to kill each of them.
Hellgate: London follows the footsteps of Diablo 2 by having glass cannons. Monsters that hit hard, but go down in a few hits.
Hellgate: London has fully randomized maps, and by maps I mean the terrain itself, not just the placement of monsters (Guild Wars had random monster placement). Unlike Guild Wars, Hellgate: London is very much about phat loot (better weapons, armor), levelling up and grabbing, learning or upgrading your skills. In short, the only similarity it has to Guild Wars is the free online multiplayer and the graphical lobbies (safe zones).
Unlike Guild Wars' multiplayer, there are planned "Raid Zones" in which up to 32 (and possibly 64) players may participate in an all out war against a gigantic horde of demons or a really powerful boss.
Remember, the game is meant to be played in single player, so there will not be any "support class" characters. Meaning, no monks. Every class is a combat class capable of kicking as much ass as the one next to him or her. You can play offline without ever having to log on the internet, much less sign onto the server, if that's all you're interested in. Likewise, you can sign on and solo, if you don't want to play with anybody else and do so only to show off your equipment, or trade items.
Speaking of which, trading will not be tedious. While it is too early at this point to say what it'll be like, an auction server similar to Sony's Station is in discussion, so you don't have to spam "WTS" and "WTB" in some common area.