Ripple may wind up being predominantly used between payment systems rather than between users directly.
Could you expand on this? I'm not sure I follow what you mean.
You think that Ripple may only have Gateways?
Payment systems may or may not be gateways, but essentially, yes. Imagine if PayPal and Western Union are both Ripple gateways. Now, a PayPal customer can make a payment to a Western Union customer through a mechanism like this:
1) The PayPal customer logs into their PayPal account and selects to make a payment.
2) The PayPal customer specifies the Western Union customer using something functionally equivalent to an email address.
3) PayPal talks to Western Union and gets the Ripple account and destination tag to use for the payment.
4) PayPal uses Ripple pathfinding to find out what the payment will cost and informs their customer.
5) The customer approves the transfer.
6) PayPal makes a Ripple payment to Western Union. PayPal decrements the balance on their customer.
7) Western Union sees the payment and credits their customer.
This allows payment systems to federate through Ripple. The payment system operators don't have to handle any cross-currency or cross-issuer issuers so long as there is sufficient liquidity through market makers in the Ripple network. The users don't have to know that the two payment systems used Ripple to make the payment.
This payment can be reversible or irreversible, depending on the agreement between the payment networks (which can be automatically negotiated at step 3). Payment systems can be payment systems themselves and/or they can accept balances from one or more gateways of their choice.
Now, you might wonder why PayPal and Western Union wouldn't just agree to do this without Ripple. And, for this example, they probably would. But if you imagine a number of payment systems using different currencies, you can see why this works better. Email used to work with ad hoc arrangements -- if you had AOL, you could use their Compuserve email gateway to email a Compuserve user. But nobody does it that way anymore, and for good reasons.
If this model takes off, Ripple's direct user base could consist mainly of gateways, payment systems, market makers, large merchants, and power users.