GirlInHat, if you would like people to have more background, and thus lift your own load a little bit by having some elements be more player focused, I'd recommend doing a few things. I've got a formalized method for determining this, but I feel like you wouldn't be interested in that. However, the basics will serve you will.
Insure each character has the following:
A flaw. Whether a personal tragedy, a terrible fear, a criminal reputation, or a powerful enemy, give your character either a weakness or something that would make people NOT want to be around them or bring them along. Overcoming flaws can be a huge boon to developing an engrossing campaign, and it sets up a lot of easy potential conflict. Can't figure out what to do with a session? Capitalize on a players weakness, and build out from there, making it a bit more personal. Note: A flaw should be serious enough that it could get the character killed or otherwise removed from the party, but not so serious that it becomes a persistent annoyance or appears insurmountable.
A purpose. This is what your characters wants, is destined to accomplish, or is seeking to avoid. It's their motivation. Before each session, I'd reccomend expanding this a bit by asking each your players (in private) - "How do you plan to pursue your purpose for this session?", which will give you lots of opportunities and ideas on how to make things personal. This doesn't even have to be desired so much as an end state the PLAYER wants their character to wind up in, in which case the question is "How will events push you towards this point?" It doesn't have to stay the same for the whole game either, so keeping up on this will help YOU keep track of how individual characters are changing over time.
With this two pieces in hand, it also gets a lot easier to trim numbers and try to come up with players that have roles and flaws that synergize well to create the sort of party arc you want to really support your campaign.
If you examine an interesting an effective adventuring party, everyone will have those two things. They are driving forces, and probably way more important than alignment. Look at something like Order of the Stick: Every character clearly has these, and that forms the core of what the story is about. Sure, it's technically about the gate, but any good game, like any good story, is really about how the character's interact with the plot, not about the plot itself.