... Oh, and while the bosses may be scripted, your teammates are not. ....
YESSS. NoFishing, this is a brilliant statement, and 100% accurate.
To answer the OP: When I started the game, I found PvE enjoyable because I liked the setting and the stories and learning the different clasess (I've now played every AC on both factions).
However, I currently find PvE enjoyable because my guild is fun to hang out with in game. We have a great time doing pretty much anything together, though we like operations the most. Heck, we'll even log onto mumble while playing our own single player games just to shoot the s**t.