Reddit reviews Inside the AS/400: Second Edition

They tend to be throughput optimised, and this is done by having more processors/code throughout the system. For example an x86 system has processor interrupts whenever anything happens in the hardware such as network packets arriving, disk transfer complete, and key presses. AS/400 has separate processors on the I/O components and they handle the fiddly work, leaving the main processors to keep going. The separate ones can be very high level - for example disk processors can do database work going off and finding relevant records, and only once done interrupting the main processor with the results.

I highly recommend reading Frank Soltis' book about the AS/400 which covers how it came about (very much a business system) and how the operating system and apps work which again are very different than x86 style hardware and software.

Edit: the hardware (and hence operating system) has more reliability features. For example most things can be hot plugged (memory, cpus etc) and there is more error checking (think ECC memory). You can do that kind of thing on x86 too, but not the commodity stuff. Another example software difference is the database being part of the operating system - not some extra component. I don't mean distributed with it, but actually part of the OS kernel.

Inside the AS/400: Second Edition.