User IDs, not process IDs. Maybe that's what you meant, but it's an important distinction. By default, no application has any access to any other application. There are IPC methods, but one process cannot scribble on another's space.
Oh, and Windows Mobile has had a horrid UI since day one. It's gotten better over the years, but it's still lousy. It's not bells and whistles; it's usability.
You can't write to another processes address space in WM. Every process has it's slot that it lives in- you can't do anything outside that slot (except for the shared memorya area), it's a simple but effective memory management system.
Agree about the UI. But also consider that you can run Flash and Java on a Windows Mobile device. Microsoft at least doesn't inject it's reiligous wars on it's users by not allowing certain things on it's platform (probably because they were sued in the past for similar behavior).