It's a matter of reasonable expectation, and structuring things so that reasonable expectation is met, for the most people, in the shortest amount of time.
For example... let's see... Roch's bard took a party to Barovia the other day. Walked all the way, RPing with these other lowbies who decided an adventure was right up their alley! Took them foreeeever to get there. All of them excited, happy to be going on some epic quest (even though, as they slowly walked along, some of them figured out that Roch was level 2 or so and were like "WTF, mate?")
Once they got their, Roch let them in on the fact that his epic quest was to fetch Grimshackle some dyes. Because Grim is a lazy bastard.
How disappointed do you think these people were? I'm not sure. If any of them are reading this, they can chime in. But I'm willing to bet any exciting expectations were dashed when they were forced to rely on their own devices, and the party began to split up. I'm sure fun was had along the way, but after a 45 minute walk to Barovia... unless you're with REALLY entertaining people, it gets dull and you start to look forward to the end, where the excitment will happen. If no excitement happens you might have had a few fun moments, but ultimately you go to bed dissatisfied. It's like watching 20 minutes of "adult entertainment" with no money shot. :-P
If you get people together for a nice adventure, it's best that - at the end - an adventure occurs. And, no, walking around in circles while members of your party slowly get bored and log other characters, or simply log off entirely, isn't an adventure. Adventure is where you GO OUT and SOMETHING EXCITING HAPPENS. Maybe in the olden days, just going out was exciting. But, once you've been around the block a few times, it's not a thrill unless something happens while you're out there... which usually requires either DMs or challenging spawns. So before I go out, I try to save myself a good forty minutes by checking around and seeing OOCly what everyone is doing, or has done. I don't want complete realism, where everyone sits around in some inn and waiting for someone to come gather a party all the time.
As far as parties go, unless there's some epic big run happening, I try to keep the party to 3-5 people. God forbid I meet other groups of people wanting to join me. Usually they just run around and get killed by accident, or they decide they want the one decent item we find, and then I have to decide whether it's worth more for me to be nice to them, or just to kill them in their sleep for it. :-P