Part of the problem with Bretton Woods was the ability to create a crisis if the values of the currency really were out of whack. A bigger problem, though, was that as a rule nobody wants to devalue their currency, no matter what the consequences. Even though a strong currency can wreak havoc on a domestic economy, very few countries will do so outside of a crisis (i.e. the UK's eventual devaluation in the 1960s). There's one exception to this (highly export-dependent countries), but then they don't want their currency to appreciate when it "needs" to for the system to remain in balance.
Bretton Woods collapsed, in essence, because the pegged values fell out of date but nobody was willing to make the changes needed to get those values back to where they needed to be to stabilize trade balances.
Edit: Per the above post...yes, I'm a mild econ geek. Some of the formulas are slightly beyond me (I hate calculus), but I'm familiar with at least some of the realities.