What are the drawbacks of putting down pavers for an alley without a sub-surface

The tannins from oak leaves will leave stains behind. I would suggest not worrying about it. Those stains are just character, part of nature. Choose to appreciate nature.

You could probably bleach them out (using simple chlorine bleach or oxalic acid), but why? Why introduce harmful chemicals into the area that may hurt you or damage your plants? Besides, the bleach may not get everything consistently the same color as it was. So you may just leave yourself with another problem to solve.

Sunlight will cause them to fade over time anyway, and as other stains occur, the pavers will become randomly stained, eventually all to the same color.

I just completed a 90 square foot patio. I ended up putting down 4 inches of paver base (crushed concrete), 1 inch of sand, and then the 2 inch patio stones... so I dug down 7 inches all around. To be honest though, I went deeper than that in some places and ended up having to bring in much more paver base (crushed concrete) than I really needed. So maybe I have 6 inches of paver base in some areas. In my opinion, the key was the compacting. I highly recommend you rent a plate compactor. You should be able to rent one for less than $100 for 4 hours and it is completely worth it. You will only use it for 30 minutes but it will make the crushed concrete as solid as rock.

Also one of my neighbors who is a landscaper joked about how far I had dug down... he said I went way too deep and now I can land a helicopter on my new patio. I tend to agree with him that the 4+ inches of paver base was more than needed but still I am happy with how it all turned out.

Couple other notes:

• After compacting the crushed concrete it was smooth and solid. When you walked on it nothing moved... kind of like walking on concrete.
• Lowes/Home Depot sell paver base by the bag but that is much more expensive than having it delivered. 2 tons (approximately 1 cubic yard) of crushed concrete was not enough to cover my 90 square feet to a depth of 4 inches; I probably ended up putting down 3 tons.
• Since the patio was positioned next to a concrete sidewalk, I used an 8' 2x4 with a piece of scrap wood nailed to one end which gave me a depth of 3 inches (the sand and paver depth). Then I pulled the 2x4 back and forth across the area (with another person holding one end on the sidewalk) to level out the paver base before compacting (and to know where the shallow/deep spots were).
• I used 1 inch diameter pipes to give me the 1-inch depth for the leveling sand. Put two pipes down on the compacted paver base parallel to each other and then pour the sand on top. Then pull a 2x4 over the pipes to level the sand. Remove the pipes and then fill in the indentation by hand. Now lay your pavers (or patio stones).
• I used polymeric sand to fill the cracks in between the individual patio stones.

Updated with photo for @Chris Marasti-Georg (2013-05-20):