As with Bitcoin, the gateways are points of failure although to a lesser degree. But there will always be a "largest gateway", or the gateway that has the largest amount of assets on deposit / IOUs issued. A gateway always exposes a surface against which government force can be used. A government can disrupt gateway activities and cause its IOUs to become worthless. This could cause a loss of confidence in Ripple in general, and a wave of withdrawals, with other gateways going bankrupt or otherwise unable to conduct business. Similar to the recent Bitcoin crash from $266 to $57.
If you have "one big gateway", it's always going to be a single point of failure. One thing Ripple does to reduce this risk is not allow gateways to freeze individual balances. So if a gateway refuses to redeem for particular people for any reason, so long as they're still in operation generally, those people can exchange balances at that gateway for some other asset. But it's certainly true that some kind of general failure of a major gateway could be a significant blow to the Ripple network. This is one of the reasons we are working to get some gateways going that are operated by regulated and insured financial institutions.