depends on the campaign, really.
on the PvE front:
some campaigns have become pretty desolate.
the first area, Prophecies, has its population centers, but most other outposts are ghost-towns. Alot of the missions (think: Instances) have few people to pick from. NPCs and newer, customizable Hero NPCs usually end up filling in spaces in alot of parties.
the second area, Factions, is pretty popular. It's also rather PvP centric, a number of missions are one faction v the other. There's a territory mechanic that can get frustrating as you pretty much have to side with one of them, and if the other "owns" the outpost, no npc's will trade with you or teach skills.
the third area, Nightfall, is still pretty busy. it's the newest of the full-blown expansions, so alot of people still running around. Introduces NPC Hero units, and as such alot of this campaign is PvE.
the "fourth" area, Eye of the North, isn't really an expansions. kind of an add-on to the first. Anyway, it feels more like WoW than any of the other campaigns. There's the standard missions, and harder dungeons (think: mid-high level Instances).
PvP wise
Nearly the entire game caters to PvP. some characters don't even touch the PvE part. there's selectable teams, PuG PvP, guilds can fight, etc.
Factions introduced probably the funnest PvP, Alliance Battles. pretty much a couple of guilds can go at it, 12v12 (3 groups of 4). pretty confusing and fun.
PvP is the heart of the game, and I got pretty bored of it after a while. i PvE and goof off most of the time nowadays, if i ever get on.
Misc stuff
Plenty of lore in the game, talk to npc's and they've got the typical auto-biography and history lesson. There's a story to each of the campaigns, but it pretty much centers around: "Oh noes, evil afoot and the world is in danger!" *your characters kill everything* "yay!"
Selling crap is a bother, no auction house.
no crafting to speak of, you collect crap and give it to crafters to make things.
equipment customizing is interesting, stick runes, insignias, and weapon parts.
the game engine does do a wonderful job at landscape lighting, probably some of the prettiest in-game vistas I can remember