During a recent playtest I encountered some serious problems with plot overload. At the start of the game I threw too many things at the player group, causing them to become confused, and sucking up far too much time dealing with the chaos. By the end I was trying desperately to draw things back and get the game back under control, but it was pretty much too late and I ran out of time to fix the problem, leading to a damp squib ending.
Point is: it’s easy to assume that the more there is going on, the more enjoyable the game is likely to be. But there’s only so much stuff a group can handle at once. As a GM whether you’re running a planned storyline or a more spontaneous, low-prep game (this was the latter) you need to think about these issues. Figure a group can maybe handle a couple of things at once, in a long-term campaign, but in a one-off or introductory session, you probably don’t even want to make things that complex. One thing at a time is probably enough!
You might be worried about the players having enough to do, and that’s a fair point. Nobody said the problem couldn’t be multi-faceted. But don’t force the players to concentrate on too many threads at once or they’ll lose the plot entirely. At best they’ll be entertained but confused; at worse, you’ll lose their interest altogether.