As alluded to, it depends on what kind of leaves: ones that break down quickly (ash and maple) or slowly (oak). Leaves that decompose quickly don't pack and prevent water absorption. In a pile, however, they will shed water in the same way a thatched roof does. Oak leaves need to be shredded.
It must depend on what kind of hay is used, if it packs or not. Here in Minnesota, I grew up tending a 100 x 60 ft vegetable garden. In early June, we laid 4-6+ inch "slabs" of hay (not straw) from hay bails over the entire garden (between the rows) that remained for the entire season. Never any problems.
