Okay, so I've noticed how people keep going into other people's common rooms. I'm in Hufflepuff, and I find it really annoying when I'm trying to work with my fellow Huffles and people from other Houses come barging in and running around like maniacs. I even know that a Slytherin set their home in our common room!
Now, I find this really annoying, and I have several suggestions for it.
#1: Change all of the Common Room passwords.
#2: Cause people to lose 5 House Points every time that they enter a Common Room that's not theirs.
#3: Send automatic mail to every person in a House every time that the House Common Room password is changed, and send automatic mail to any new players after they find out which house they're in about what their House Password is.