What an interesting thread!! In these situations I will take my plate/container back, but would never take home any of it unless I was asked by the host. That being said I too get secretly cross if my plate hasn't been served. Just seems like a waste of time/effort/money then.
I'd rather organise the food myself when I'm the host - too much of a control freak to leave it up to everyone else But would offer the leftovers back if anyone brought something, just as a courtesy.
The only story I can think of with this fuzzy baby brain is ages ago we had a little get together. One girl is a huge Coke drinker & so we had bought lots of Coke. She brought one bottle with her, fair enough. But when she left she took a bottle with her, despite having drank loads of Coke!! I understand taking home your alcohol - but Coke when you have been drinking the host's all night?? I didn't say anything, if it's that's important to her she could have it!!

I agree - an etiquette section is a good idea.