A week or two ago, I finished running a VtM 20A game where I had the beginning planned and the ending planned, but the middle was mostly mush. The players all had various goals that they wanted to accomplish, and I had to attempt to make sure that everyone got those goals fully done around the same time. Although everyone seemed to have a lot of fun, I'm not sure that plan worked perfectly.
The rough part with ending a story is that it ends. A lot of people don't like that. They want the story to keep on going, develop new quirks, find snags and continue on. For a GM, this can be difficult because you do eventually run out of ideas and you want the game to end on a memorable climax.
The really difficult part, as far as GMing when compared to solo games, is that it's much easier for the two people involved to be in agreement during a solo game. In a solo game, people will talk beforehand and come up with plans, but in a group game, especially a system game, people want to be surprised and don't want the entire story laid out before them. This means that if you want to "nudge" players into a course of action, you have to be exceptionally talented at it in order to avoid the feeling of railroading.
((If you ever absolutely have to railroad, convince the players that they got one over on you. If you can succeed, it'll feel much less contrived because they'll figure that they did what was obvious.))
Thus, a group game is likely to go very off course as it continues along. The players aren't going to want to drop the Seven-Sided Star into the Bottomless Vortex. One of them will want to keep it in her tower, another will attempt to siphon off its power, a third will attempt to eat it, and only one will say, "Hey, guys, this thing is pure evil, so we need to...guys?"
I suggest preparing up to the limit of what you're able to foresee, but allowing for changes. This is how I'm doing my second novel, after with my first one, I just started writing from scratch, then rewrote the thing and started tying everything together. Of course, that's not entirely true, because I did have the basic plans of each "Act" for the first one which I followed out...I think. It's been a while, so can't 100% remember.
When running a group game, I know a guy who writes up the PC-less story. If the PCs don't intervene, the Evil Overlord will claim the Ring of Fire, all of Gorgotosh will be destroyed, and the few remaining elves will be hunted down like wabbits. This can work in theory because the players will be sent along that path, but we all too often ended up getting delayed, side-tracked on things and such, and would end up running out the clock before it was too late to act. Then, we're suddenly in a less fun situation where the entire High Castle is engulfed in flame and impossible to breach, and we're fighting for our lives against superior numbers in a losing battle, while he tries to figure out what went wrong.
If you can convince the players that they always arrive just in time, awesome. Like a video game, it can get too contrived if the players know that there's no time limit, and like a video game, they'll want to wander off and do every single side quest before the end. It breaks the feeling of being rushed and imminent danger when they know that the BBEG's machine won't hit the five minute mark until they bash in the door, but that doesn't mean you can have them show up too late, either, unless you're fully planned and prepped to make such a story fun.
I really got to rambling on this post. Sorry about that.
I'm going to say, there are active players and there are passive players. Active players, in a sandbox situation, will have little difficulty in finding their own tasks, making friends, putting things together and so on. A GM doesn't have to worry about inventing a story, but only has to worry about dealing with the NPCs, having everything work out, and inventing the occasional wrench for the plans of an active player.
A passive player, by contrast, will just show up and follow along for the ride. A passive player is used to simply being told the story, having the town sage point them in the right direction and marching down the path. You can railroad a passive player with a linear plot straight out of a video game, and they won't care as long as it's interesting. Their characters won't question the king's motives, so they won't ever consider that maybe the king can't be trusted with the Staff of Ultimate Power, unlike active players who will want to either destroy it or keep it for themselves and start their own kingdom.
These aren't absolute either. I'm probably 1/3 Active, 2/3 Passive. Most likely there's some triggers for what makes people active or passive, but I don't know what it is.
On Elliquiy, you have such a vast number of players that you can pick and choose to some degree, sticking with the kind that you like. If you love huge stories that you invent beforehand, passive players are ideal. If you just want to be spontaneous and run for people with plans, then active players are for you.
Okay, that's probably enough. Sorry for rambling.