I'll contribute to the idea of having a container on the rear rack- I have a green Sterilite crate bungied to mine. I really like the convenience of being able to reach back and grab my washrag handkerchief while I ride. The crate holds a fair amount of stuff- usually just my purse. I have a PSP case strapped to the left side inside it to hold my multi-tool and flat patches and such.
With heavy items, my bike definitely required careful handling. A car battery is heavy. A 5-gallon container of gasoline is heavy and awkward from the liquid sloshing around. You need to find a place other than your seat post to mount rear lights. Also heavy items in the crate way up high just BEG for a better kick stand. I really want a double-leg one, like Pletscher's. I had to lean my bike very carefully against things. Stopping to wait for traffic, then taking off again required some practice, and was slightly scary.
I think I really want folding baskets, they seem like a better solution- closer to the ground, conveniently out of the way when not needed, and allows storage across them and the rear rack if even more space is needed. But, they are more expensive than my $3 Walmart crate.