Minecraft – Are there any advantages between cobblestone wall and regular fence

Update: According to Notch's blog post about changes coming in 1.6, the torches-on-fast-leaves bug is getting fixed:

• Fixed buttons and levers being placeable on leaves in “fast graphics” mode

This means that monsters can no longer spawn on leaves, either, as they are always considered transparent, even when using Fast graphics.

Monsters can spawn on any solid, non-transparent block which is at light level 7 or lower.

Signs, torches, rails, and the like are ignored when spawning monsters; they'll spawn on the block underneath just fine. This is why one-block-wide railways need to be lit.

Transparency means whether or not light passes through a block rather than whether you can see through it. Half-blocks, doors, stairs, and glass all count as transparent and prevent monster spawning. Leaves, oddly, count as transparent only when Fancy graphics are enabled. An easy way to test a block for transparency is to try placing a torch or sign on it — neither can be placed on transparent blocks. (Meaning that torches can be placed on leaves with Fast graphics, but not with Fancy.)

Torches put out light level 14, so monsters can't spawn within 5 blocks of a torch orthogonally or 3 blocks diagonally. If you put your torches in a diamond pattern, they can be up to 9 blocks apart, but it's best to move them closer than that on uneven terrain. (I use 5 myself.)

If you like, you can actually torch your treetops by changing your graphics to Fast, placing the torches, and switching back. Your torches will stay rooted, even though you wouldn't normally be able to place them that way. Personally, I think it makes them look rather cheerful. :-)

It's not possible for snow to exist on slabs/half steps. A slab is only ½ blocks high, but as far as the game is concerned, it inhabits an entire block. The same goes for the snow layer, which is also a block — except other blocks you place can go through and replace the snow layer block. So snow on a slab would have to float half a block above ground.

Try placing a cobblestone block on a slab, same thing:

Double slabs are a different block entirely. Their height is exactly one block, so snow can exist/will fall on it.

Edit to add:

Are there any solid blocks that snow cannot stick to?

Those would be non-opaque blocks: Glass and ice. In Fancy Graphics mode, snow will also not fall on leaves.