They appear to have considered the obvious things: write and execute access to files and other resources from restricted processes, preventing harmful windows message behavior.
But would a process with low integrity have read access to every other resource with low integrity? Or is that further constrained by some other access control mechanism? Also, I don't think it's a feature for an unprivileged process to have registry access, though I understand there may be reasons for it.
It could also be argued that there is a larger attack surface for protected mode processes, because they can call any OS function, but are just denied when they don't have the access level to commit the action.
I must admit I am behind the times with regards to Windows system programming.
I won't attempt to explain the integrity system in Windows because I am not that familiar with it. But, Wikipedia says that read access can be blocked based on integrity level too. See http://en.wikipedia.org/wiki/Mandatory_Integrity_Control