Mosso is one of those used-car-dealership services. They insist that you're getting something amazing, but they won't exactly tell you what you're getting.
More importantly, their $100/mo plan includes 3,000,000 requests - that's not a lot considering that every CSS file, image, page, javascript, etc. is all another request. So, they're giving you a server that can handle 1.16 hits per second - DreamHost could do that!
Let's say you have a nicely busy site with 20 hits per second - your next big thing. That's 51m hits/mo. You'd be hit with over $48,000 in overage charges for requests alone - nevermind bandwith overages. Even if your site is a small next big thing and has 5 hits per second, you'd still be hit with nearly $10,000 in overage charges.
Plus, do they really guarantee automatic scaling? No. They're not going to auto-scale a twitter or reddit. You're buying the idea that you never have to think about server administration and that it'll come cheap.
Amazon has a realistic offering. Mosso is a way for you to tell people that no one has to worry about scaling until it hits the fan and you're finished.
If you disagree, maybe you could shed some light on Mosso. Do they automatically set up mysql clusters - multi master and all - as you get too large for one database server? What if I decide to write a page with a looped query: do they just scale that? What if I'm joining 20M records against 10M users (an expensive, On^2 operation)?
There's no such thing as a free lunch.