One thing I thought was important that they didn’t touch on is why IaaS matters for some scenarios.
Richardson only really talks about how IaaS can be more flexible and have less lock-in (because you can install any OS or software).
but thats not the whole story..
IaaS is particularilly useful if you are planning to virtualize an existing server infrastructure to the cloud
which is where it really shines.
Knowing that, the key point to selection really should be about what level of abstraction you can deal with. If you need control of the OS and installed applications, you need IaaS (AWS).
If you don’t need control of the OS, you should chose PaaS (Azure) because it reduces the number of things you are responsible for maintaining.
No OS software updates or configuration to worry about (fewer moving parts).

Both platforms have similar levels of lock-in (which was a big topic)
If you stick to just creating virtual instances in AWS and don’t use any of the data storage services, you have essentially no lock-in (assuming you aren’t using their web services for scaling the number of servers either).
Same with azure, if you only use the web modules, worker modules, and SQL server, you can pretty much take your app back to IIS any time
Once you start using the blob storage and app fabric services, you are somewhat tied to azure
I think they put too much weight on that issue.