The Great Depression was made so much the worse by the terrible policies that followed (Hoover's first, then Roosevelt's). In particular: the failure to "liquidate, liquidate, liquidate" (as Andrew Mellon wanted in '29), the subsequent insufficient devaluation (Hoover didn't devalue; Roosevelt did but not enough) and the banning of gold (while notionally retaining a gold standard for trade) -- this killed international trade, perhaps more so than Smoot-Hawley. Insufficient devaluation kills imports, but when international trade is settled in a currency not your own (e.g., gold) it also kills exports! You can't neither liquidate nor devalue. Add to all this labor market rigidity (e.g., brow-beating companies into not lowering wages led to inability to hire) and other terrible policies, and the 30s were a mess.
The 2008 crisis too was followed by bad policies everywhere (e.g., competitive currency debasing and reflation attempts, QE, ...), but nowhere near as bad. In particular there was no gold standard, currencies were mostly floating, and no attempt was made to shut down international trade. Not that a gold/whatever settlement system is necessarily bad, but that floating currencies made it impossible to insufficiently devalue, as Roosevelt did.
We seem to have learned some bad lessons from the Great Depression (Keynesian economics good), but also some good lessons as well (trade good). We have also seen Rooseveltian policies applied elsewhere since then, always with terrible results. E.g., insufficient devaluation / trade killing policies have been applied a number of times in Latin America, only there the U.S. dollar plays the role of gold.
Perhaps the best way to think about credit crises is to think that the aftermath lasts as long as it takes to clear bad debts, including the processes of repossession, plus the amount of time it takes new owners to return their acquisitions to profitability. Get debtors' mothballed assets back into production by liquidating them.
Deflation, by the way, is good and unavoidable in a credit crisis. It speeds up the process of identifying and working out marginal and bad debt. It helps the poor. It helps prospective and actual new owners. The worst thing that can happen in a crisis is for assets to become non-productive and under-maintained for so long that they effectively rot/rust away and become unsalvageable, and this is what happens when governments and banks extend-and-pretend.