What are Ripple's weaknesses? Bitcoin's vulnerabilities are well documented. Is there are similar document for Ripple?
These are Ripple's largest known weaknesses (not including weaknesses common to Ripple and Bitcoin):


Because the Ripple network can't move actual fiat currencies, balances in Ripple act like bank balances. If the bank somehow defaults on its obligation, people won't actually have the money they thought they had. This is not a big problem if you're just using Ripple to make payments, but it's a weakness if you're using Ripple as a store of value for fiat currencies. Your money is only as safe as the "bank" that holds it for you.
If some unified group were able to get control over a significant number of validators that other validators trusted, they could temporarily disable the Ripple network (by refusing to reach a consensus) until manual intervention by other validators removed them.
Ripple's internal mechanics are still largely untested. As happened with Bitcoin, problems may be discovered as people begin to heavily use the system. Ripple's scalability has only been tested in simulation.
While the hope is that gateways will be all close enough to equal that people will be able to just sum their balances to get a sensible total, it's possible that different redemption policies or perceived trustworthiness will result in some balances being "devalued". So you might deposit $10 at a gateway, get a "$10" balance in Ripple, but find that because your gateway is not considered reliable, nobody actually values that balance at $10.
People might get tricked into extending trust to entities that are untrustworthy. We are already seeing "you trust me and I'll trust you" type offers. People may not realize that the system will give them what they ask for, giving them balances they have no way to collect on. Essentially, Ripple simply records balances. The system can't force people to settle them outside the system.
The stability and possible future changes in the value of XRP is unknown. If XRP is usable as a bridging currency, that should make cross-currency payments cheaper. But there's no assurance that it will be.
Until the network is decentralized, OpenCoin is a single point of control and possible failure.


In exchange, Ripple provides a way to perform fast (ten seconds), cheap (less than a penny) cross-currency payments with no way to lose money unless someone you chose to trust defaults. Ripple provides decentralized exchanges between any number of currency pairs. Ripple provides a way to seamlessly federate payment systems so people who use one payment system can pay people who use another, even across currencies. Ripple's design requires no central authorities and OpenCoin is committed to making it an open source, decentralized payment network.