One of the beauties of any novel is how the different characters and their stories mingle with each other, but it's easy to leave one (or more) characters in the dust.
While stretching out the story of one dude may get you on something of a roll, I can't forget to let the other guys (and gals, for that matter) catch up with their stories, too. I'm writing in the first person, so that takes some of the sting away, but there are still things going on.
You can't set up a meeting that has to happen quickly between two characters, and then return to it 15 chapters later. Well you can, but that's not something I want to do. Readers aren't stupid, and they're not going to forget things, but if one character says "I can't wait to talk to you about" something, then he/she really can't wait. You gotta make it happen while all the other stuff is going on around them.
As a newspaper guy, the 'Point A to Point B' has been my business for the better part of the last two decades. One story. One string. With the novel, now I have a lot of strings. I'm pulling them all in the right direction (I hope), but I gotta make sure I'm pulling all of them. I don't want to accidentally drop one.