I know I haven't played regularly for some time, now, and this might seem like it's out of place, but I'd just like to say I agree with Cruzel on one of his points there.
That point is that of the little nudges. Beyond just making a quest more challenging (that works too), giving the player those small hints, really simple events that might motivate them to do things they did not expect to, small situations in quest with the potential to generate conflict, that really simple mission of "go look for the generic orc/slaver/bandit/seeker who's been up to something."
Those little things are an amazing way to get people to be proactive and involve other people, when they usually wouldn't. I'd suggest doing something like that every once in a while, if you don't do that already.
That point is that of the little nudges. Beyond just making a quest more challenging (that works too), giving the player those small hints, really simple events that might motivate them to do things they did not expect to, small situations in quest with the potential to generate conflict, that really simple mission of "go look for the generic orc/slaver/bandit/seeker who's been up to something."
Those little things are an amazing way to get people to be proactive and involve other people, when they usually wouldn't. I'd suggest doing something like that every once in a while, if you don't do that already.