The campaign is more like "go to this location and kill this person" with a lot of killing along the way. Repeat.
That is the entire game. Though person might be a stretch.
From what I can tell, all the side quests are fetch quests. Better than go to this region and kill things until you get drops. Easier for lots of people to do at once.
So far only part of game that is single player was the 1st area. Dungeons are instances so they should just have you in them. Town is where I see more. Sometimes see people in wilderness.
Diablo has always been more multiplayer focus than single. At least, from my perspective. So it doesn't need to become like wow. Its main towns aren't designed for the crowds you could fit into Orgrimmar or whatever Alliance towns were called... never played them. Sacred 2 was the same, more weighed towards group play.
PC Gamer should do an article on the Memory leak as it might get Blizz to fix it if its being talked about, People on reddit thought it was GPU drivers but it happens with both AMD & Nvidia cards, so that is unlikely. Part of the game is asking for commitments and not giving the memory back when finished. You can run out of memory and still have ram, as it chews up Virtual memory, which is page file. It grew my page file from 5gb to 16gb yesterday. The most ram I saw in use by game was 17gb, I have 32gb so it should have had plenty to spare.