Last year I bagged some of the leaves in my yard and spread it about 4" deep on an area by my mailbox. It held the weeds down fairly well. The ones that did come up were easy to pull out. Most of the leaves were dark red so for a short time it had a really nice color to it.

 

Yes, as long as they're deep enough, leaves or grass clipppings work great as a weed-prevention mulch. Decades ago, when I bagged grass clippings and grew a vegetable garden, we'd use grass clippings and leaves as a weed-prevention mulch. They work great and help build up the soil.

 

+1 ken-n-nancy. I've also done this for many years. Also, the leaves seem to inhibit weed germination, to some extent. Hopefully, the link I attached works. There is a pdf download too.
http://horttech.ashspublications.org/content/19/2/297.short

 

If you get them thick enough and then wet them down so they won't blow off then yes, they will prevent weeds, grass and most anything else from coming up. I generally chop my leaves up into the lawn but I do blow some of the leaves in my back lawn into a "natural area" for this reason. There are a few stumps and roots in that small area that make it difficult to mow plus most of the time the dogs do their business there too. Over the years it will also greatly improve the soil.

 

One problem with leaving leaves in the beds is they become a breading ground for ticks but if you chop them up or much them, it should work good.

Now for lawns, chopping up the leaves and leaving them is a good a weed and crabgrass preventer. Here is a study someone shared with me.

Good read.

http://www.canr.msu.edu/news/to-bag-or-not-to-bag