The real estate market can be visualized as a massive pyramid. There are very few multi-million dollar properties at the top of the pyramid, and a large number of relatively inexpensive entry-level properties forming the base. Like any structure, if the foundation is weakened, the structure may collapse. In the same way, housing markets collapse from the bottom up due to problems with affordability.
The foundation of a residential real estate market is the entry-level buyer. Entry-level buyers are generally young people starting to form new households. When homeowners want to sell their house and move up to a nicer one, someone needs to buy their house. If you follow this chain of move-ups backward, eventually you come to an entry level buyer. If there are no entry level buyers pushing the sequence of move ups, the entire real estate market ceases to function.
The entry level market was initially boosted the moment 100% financing became available because many more people were enabled to purchase; however, it was imperiled at the same time because of the change in savings incentives. This market was subsequently destroyed the moment 100% financing was eliminated because few entry-level buyers had a downpayment and very few people were in the process of saving to get one.
In the past, people would rent and save money until they had the requisite downpayment to acquire a house. The barrier to home ownership was not the ability to make payments; it was having the necessary downpayment money. When downpayment requirements go up, the number of people capable of buying a house declines considerably, particularly for entry-level buyers who must save this money rather than transfer it from a previous sale. Since few potential entry-level buyers were saving money during the rally, sales volumes suffered dramatically in the wake of the bursting real estate bubble.
The weakness in the base of the housing market is going to serve as a drag on sales and pricing for many years to come. The government will likely come up with some artificial stimulus to encourage sales, but once the stimulus wears off, the same underlying problems will resurface. We must rebuild the base of the housing market through savings and stable financing. This will take time because the bad incentives and practices of the Great Housing Bubble.