0. "Trust" (confidence, fiat style) instead of decentralized & provable. Saw that you have 1 BTC on Ripple? You don't. You are TRUSTING that someone has 1 BTC 'for you'. Compare this to a true cryptocurrency - you have 1 bitcoin, you can sign the private keys to spend it. Not someone else who is bribed by Ripple (see #4).
1. Not open source. You can't get the server source code, make changes to it or create your own fork.
2. 100% of the XRPs (which can be traded with btc, usd, etc) are premined, and goes to the founders and OpenCoin Inc.
3. Centralized as only OpenCoin Inc developers can contribute to the code and make whatever changes to the network as they desire.
4. Bribes exchanges with XRPs (NovaScam style), exclusive benefits, so they accept Ripple and otherwise won't have.
5. Distributes small coins to people to encourage usage of them. They plan to only distribute half of it, and keep half it it themselves to dump & bribe people (see #4)
The idea behind Ripple is certainly interesting and novel, but with the current business decisions Ripple is a centralized, centrally issued and non open source scam currency.
This is not a scam in the strictest sense of the word, but rather deceptive business practices that are attempted to be kept secret.
Original thread: https://bitcointalk.org/index.php?topic=147789.0
