The really big change, I think, is that Ken Lewis has explicitly stated that a BofA-owned Countrywide will not make any subprime loans. This means that for subprime borrowers, it's going to be harder than ever to find a mortgage. And for everybody else, less competition in the mortgage-origination sector is likely to mean higher mortgage prices. The combined company will have 25% of all mortgage-origination business: in the UK, that would officially make it a monopoly.
Now mortgages are sold almost entirely on price: borrowers don't care who their lender is, so long as they're getting the cheapest possible rate. So BofA-Countrywide isn't going to have a huge amount of price-setting power. But at the margin, I think mortgages are going to be harder to get and more expensive in the wake of this deal.
On the other hand, existing homeowners whose mortgages are being serviced by Countrywide should probably be breathing a sigh of relief. No one wants a bankrupt loan servicer, and now they know that isn't a risk any more.