From a technical POV, there are two main versions of contactless payment cards - For MasterCard there is PayPass M/Chip and PayPass MagStripe. I believe the Mag Stripe version may just be used in the US, where there are fewer chip cards, while M/Chip is used on cards which have EMV chips. (ref)
I believe the current versions of PayPass M/Chip do perform crypto on the chip and produce dynamic hashes, meaning the transactions can not be replayed. This value is called CVC3 and can be static or computed dynamically depending on your issuer. (ref) I think dynamic is more common now, but I'm no expert. Naturally, only dynamically generated CVC3 values can't be replayed.
I've heard plenty of people ask the question about someone in a crowd with a PayPass terminal. I can't believe any merchant would allow this to happen - because MasterCard and VISA and probably their bank, would be very, very upset and would shut the merchant down quickly. As the fraud is being performed by a merchant, not a customer I think it would be found and stopped quickly. Maybe I'm naive, but it seems this would be a terrible method of committing fraud.
From a banking point of view, in Australia at least, provided you report your card as lost or stolen, your liability is limited to some nominal amount (I am not a lawyer - consult your bank). The transaction amounts here are limited to $100. I assume banks / MasterCard / VISA have run the numbers and still feel the possibility of more transactions and associated transaction fees, outweighs the risks.
Additionally, we're also now seeing small value VISA transactions not require a PIN or signature - so this kind of issue now exists for non-paypass transactions.
Regardless of all this, I'm pretty sure VISA and MasterCard will at some point mandate the inclusion of PayPass/Paywave in newly issued cards.