One thing nobody's named in the thread yet: WASM's validator is linear-time, single-pass, with no dataflow joins. That constraint is what gives the operand stack its weird shape. Every block, loop, and if carries a function-type signature. The operand stack at block entry has to match the parameter type, and at exit has to match the result type. Inside a block the validator only sees pushes and pops within that frame; it never has to merge stacks from sibling control-flow paths, because each path's exit type is independently checked against the same expected signature.

JVM went the other way: arbitrary control flow plus a verifier that does dataflow with type merges across joins. That's expensive enough that JVMs do it once at class load and cache the verified state. WASM specifically didn't want that bill; fast startup was a hard requirement.

So the prefix/postfix debate elsewhere in the thread is downstream of this. The encoded form is postfix because that's what trivially admits a linear validator; the textual LISP form is sugar for the same expression trees inside a typed frame. dup isn't missing for aesthetic reasons either: local.tee n followed by local.get n already gives you dup-equivalence through typed locals, and any stack op that didn't reduce to typed locals would either duplicate what locals already do, or break the validator's linearity guarantee.