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The 90/10 Rule
The x86 architecture neatly embodies the saying that 90% of your users will only use 10 percent of your features. The nice thing about x86 is that, whatever you want to do, you will probably find some previous designer. Much of the time, unfortunately, you will then find that AMD removed it with the Opteron, and it is no longer present.
May vestigial parts of the instruction set remain, such as string and BCD instructions, and others still retain bizarre limitations. The compare-and-exchange instruction, for example, takes two operands, and uses EAX as an implicit third. This is fairly common, although the implicit operand varies between instructions, making it quite difficult to make efficient use of the small number of registers provided by the system.
It is a quirky and idiosyncratic architecture, with some good points and a lot of ugly ones. Unfortunately, until someone starts making cheap SPARC laptops, we're stuck with it. The good news is that compilers and high-level languages mean that, most of the time, we can ignore it.