An interesting note, though, that the players didn't even pick up on was that I didn't start the session with stat blocks for the spiders. I knew I wanted the small spiders to use webs to trap the PCs and pull them closer, and I wanted the large spider to be a more significant melee threat. With a couple of notes on those abilities, and using a generic set of level-appropriate attack bonuses and defenses, I was able to generate those monsters nearly entirely on the fly.
Of course, it helps that I'm currently very familiar with the structures we are using for building monsters - but to me it highlights what is important about a monster in the context of an encounter. Generally, you need to know numbers for hp, defense, attack, and damage, and then you need the one or two special abilities that make the creature unique in combat. It seems obvious, really, and wasn't a novel realization, but was just something that I was thinking about last night.
(And yes, I know that many monsters need some interesting things to do outside of combat - but that wasn't information I needed during those encounters.)