The Meaning of Life, the Universe, and Everything.
I think that slabs should me able to mixed, like if you have a slab floor, you should be able to place another slab type right on it and it would only take up one block of space. This may make world saving a little more complicated, but you could then do many new designs and have better looking buildings. Just a suggestion.
"Colder than the snow that blankets the earth" "Colder than the ice that cover the stones and freezes the rivers" "Colder than the north wind blowing through a winter-bound forest" "Colder than all these is the smile of a Shade of the Dark"
This has been suggested so many times, I can't even begin to count them.
This is a definite no, because Mojang would then have to create a new block for every single combination possible in terms of top/bottom for each slab. That many blocks would seriously slow down minecraft, and would add only a minor feature.