IBM ROMP vs. ARM
The IBM ROMP chip (aka the 801) was never intended to be a general purpose RISC processor. It was intended to power an office automation product (think of a hardware word-processor like WANG used to sell).
As a result, although it could function as a General Purpose CPU, it was not really that suited for it. It was never a success because at the time, IBM could not see justification for entering the pre-Open Systems UNIX world. RT 6150 and the 6151 were intended as niche systems mainly for education, although they did surface as channel attached display front ends for CADAM and CATIA run on mainframes (and could actually run at least CATIA themselves). This changed completely with the RIOS RISC System/6000 architecture, where IBM was determined to have a creditable product, and invested heavily.
In comparison, the ARM was designed from the ground up as a general purpose CPU. Roger Wilson (as he was then) greatly admired the simplicity and orthogonality of the 6502 instruction set (it is rather elegant IMHO), and designed the instruction set for the ARM in a similar manner. Because the instruction set was orthogonal (like the 6502, the PDP11, and the NS320XX family), it makes the instruction decoding almost trivial. It also made modelling the ARM on an econet of BBC micro's (in BBC Basic, no less) much easier, which allowed them to debug the instruction set before committing anything to silicon.
They had to make some concessions on what they wanted. There was no multiply-add instruction, which appeared to be a hot item in RISC design at the time, and to keep it simple and within the transistor budget, all they could do was a shift-add, (the barrel shifter), which although useful, was a barrier to ultimate performance, but great for multi-byte graphics operations.
It was also simple enough so that they could design the interface and the support chips (MEMC, VIDC and IOC) themselves, achieving early machines with low chip counts.
This is all from memory of articles in Acorn User, PC World, Byte and other publications. Feel free to correct me if my recollections are wrong.