Humidity and warmth are some key factors. Soil does have some benefit allowing the local environment to stay more humid for longer periods. And, I find leaves on the floor of the greenhouse all the time that have roots and new growth. Having said this, I have some Echeveria leaves that root and never grow new plants no matter what i do. In fact, I just tossed several different hybrid caruncled leaves that had huge root masses but no growth.

I've read on some websites that they let leaves callous over for a few days to a week and also wait until they start growing roots before putting them on top of soil. I've tried that with a few of my leaves, but I found that the roots didn't grow until I put them on top of the soil, spritzing the soil with a little bit of water every morning after they calloused over. It might depend on the humidity of your location, though.

It's also best to leave them in the shade with indirect sunlight. Apparently leaving them in the sun is too harsh - I've had a few things dry out too quickly in the sun. Mine usually sit in a north-facing window.

The leaves off the bloom stalk are even easier to root. I often just put the leaves under the plants skirt and then check on them later.

Thanks for the replies guys. Its good to hear that soil is not needed for rooting; however, I had no idea that the leaves could grow roots and no resulting plant. I'd say average relative humidity here is at least 50% or higher, not too bad I guess.
I guess I'll just leave them in shade for a while see what happens. I will definitely keep them out of the sun, I have fried enough plants to learn my lesson lol. It would be nice if it flowered, as it hasn't yet, but I'll be equally as happy if these leaves take off.

Here is an example of leaves that I've had rooted for more than 6 months. I pitched them because they aren't growing any plant, yet the leaves are firm. These are only some of the leaves I've recently tossed out. Don't know why but I do know some Echeveria will not grow new plants from leaves.

Thanks for the picture bikerdoc. They look good, but yeh so weird as just a solitary leaf with roots.

Any idea how long it might take for roots to begin showing?