Yep, I'd start packing things away / cleaning, unless they said it was okay, at which point I would double check
I would expect the same in return, or at least an offer (even if you are going to say no, it's still nice to know someone is willing to help)
From the belief that you treat others as you wish to be treated
I feel the same way... I like at least to have the option to say no.

If you are hosting it, maybe you could try taking the initiative? When it is coming to an end and people are getting ready to start going, get up and say " OK everyone, lets all help tidy the toys away." You can direct this at the children, but loud enough and clear enough that the adults pick up on it too. If they still don't help, just say quietly to them "Oh, do you mind picking up x, y, z before you go?" Most adults wouldn't have any problem with that.
I'll give it a go
I've gotten to the point that it depends on where I am as to if I bother.

I've always been one to help out - we don't go to MG, but at family, etc. I do dishes, clear tables, tidy toys, etc. But when the people I help out don't bother, and leave me with a bomb of a household (we keep tidy over here) and a sink full of dishes, I stopped doing it when I went to their place.

It annoys me too because the people I'm talking about all know that I keep things tidy, yet they never even offer a hand. I have the same thing with bringing a plate. I never ever ask anyone to bring anything, but am always told what to bring, specifically, and it's usually an expensive dish too.
Maybe I should try that at thier house. Then again..my conscience would get the better of me. I totally get you on the food thing too!

We have playgroup at a local church so we all pitch in and pack up and take turns to vacc and sweep. It's done very fast when everyone pitches in.
We do this at playgroup too!