1) What if a huge gateway (such as Bitstamp) just decides "nah we're not going to do Ripple anymore"...and all their IOU are now worthless; it would bring the whole thing down, everyone would be trying to unload their worthless balances, it would be total melt down. And the best part is Bitstamp could probably continue to operate as a regular exchange (what is Ripple going to do about it)
Their clients would sue them and win.
2) What if Opencoin starts leaking their billions of XRP onto the market to try to turn a profit? As soon as people start to notice everyone will dump and run
If you're worried about that, then don't invest in XRP. You only need tiny amounts to be able to use the Ripple functionality for other currencies, including BTC. And of course, OpenCoin has no motive to do this, because they would be shooting themselves in the foot if they did. They want the XRP exchange rate to go up, not down, just not uncontrollably.
3) What if Opencoin get shut down by the government?
The protocol is distributed, just like Bitcoin, so governments cannot really shut it down any more than they can shutdown Bitcoin or Bittorrent.
4) What if Opencoin gets hacked and hackers use their Opencoin servers to do all kinds of bad stuff
5) What if Ripple itself gets hacked (source is still closed so no one knows how secure it is, there may be major security flaws)
6) What if Opencoin decides to randomly start doing bad shit like making more XRP, confiscating balances, banning accounts...etc..
Only a problem until OpenCoin releases the source code. It is in their interest to do so, precisely because it will dispel fears like that.
These are just a few glaring "What if"s, I imagine people with more knowledge on the subject could come up with 20 more than this.
Basically Ripple makes Opencoin the de facto Fed...
Scare mongering.