You have to remember, they are a Y Combinator funded start-up and this is a Y Combinator site, so there is a fair amount of hype on here about Stripe.
That said, I think they are doing something interesting and useful (in a real-world commerce sense), and wish them well with it.
I guess their principle differentiator is the market they target - but pitching to developers is a big bet. If you've worked around enough enterprises you know by now that developers have little say in business decisions - meaning that their main opportunity is to be chosen by startups, and to hope to high hell that one of them makes it big, and piggyback - and hope to high hell that a big player doesn't use their clout and cash to undercut them and shoulder them out.
Either way, kudos to them for building what's undeniably a good product, but I just don't see it as a gamer-changer in the same way as others seem to.
Pricing model - no, it's great, but as you grow, it becomes uneconomical.
What's stopping them? Fraud. Fraud fraud fraud. They will end up with an approval process, as if they get hit big a few times (they will), their merchant acquirers/clearers will insist on it.