My MacBook seems to be an insomniac (won't sleep)
I found my answer here.

To sum up:


Open a terminal
Run "pmset -g assertions"
Check the output if there is any process preventing the system from sleeping.


In my and apparently many people's cases, it was actually an unused printer driver that seems to be in "in use" state. I deleted that printer driver (not all of them!) and my system now goes to sleep peacefully after closing the lid (even on power).

Lion seems to keep the system awake if the power is connected and there are any services or devices shared to others.

Btw, I am still getting that sound assertion failure when the system goes to sleep.