> 16-bit instructions, and instructions that only specify two registers rather than three

These are not strange design features. I'm surprised they were patented? For example, most non-SIMD x86 instructions are two-operand. The trade-off between two-operand and three-operand has been known for a long time. (Shorter instruction encodings, but sometimes you need more instructions)

I actually didn't know the THUMB thing; pretty interesting stuff...

The branch delay slots/load delay slots thing was also something that plagued some MIPS processors back then too; 5th generation of consoles (At least, from Sony, Nintendo and Sega) had either all or a subset of those quirks on their flagship machines.